الوظيفة الشاغرة في الرسالة.

It is most naturally understood as a noun phrase, not a full sentence.

Literally, it is something like the vacant position in the letter or the job opening in the letter.

Arabic often uses short noun phrases like this in:

• headings
• captions
• labels
• exercise prompts

If you wanted a full sentence, you could say something like:

• هناك وظيفة شاغرة في الرسالة = There is a vacant position in the letter.
• الوظيفة الشاغرة مذكورة في الرسالة = The vacant position is mentioned in the letter.

الشاغرة means vacant, open, or available.

It is an adjective describing الوظيفة.

So:

• وظيفة = job / position
• شاغرة = vacant / open

Together:

• وظيفة شاغرة = an open position / a vacant job
• الوظيفة الشاغرة = the open position / the vacant job

Because in Arabic, an adjective usually matches the noun it describes in definiteness.

So if the noun is definite, the adjective must also be definite.

Here:

• الوظيفة = the job/position
• الشاغرة = the vacant

That sounds strange in English, but in Arabic it is normal adjective agreement.

Compare:

• وظيفة شاغرة = an open position
• الوظيفة الشاغرة = the open position

Because الوظيفة is a feminine singular noun, and Arabic adjectives agree with the nouns they describe in:

• gender
• number
• definiteness
• case

Since الوظيفة is feminine singular, the adjective must also be feminine singular:

• شاغر = masculine singular
• شاغرة = feminine singular

So you say:

• منصب شاغر = a vacant post
• وظيفة شاغرة = a vacant job

في الرسالة is a prepositional phrase meaning in the letter or in the message.

It tells you where the job opening is being mentioned or found.

Breakdown:

• في = in
• الرسالة = the letter / the message

So the whole phrase means something like:

• the vacant position in the letter
• the job opening mentioned in the letter

Because في is a preposition, and in Arabic prepositions are followed by a noun.

So:

• في البيت = in the house
• في المدرسة = in the school
• في الرسالة = in the letter

Also, after a preposition, the noun is grammatically in the genitive case.

In fully vowelled Arabic, this would be:

• في الرسالةِ

But in normal writing, the final case vowel is usually not written.

A careful full pronunciation would be:

al-waẓīfatu ash-shāghiratu fī ar-risālati

In normal pause, learners often say it more simply as:

al-waẓīfa ash-shāghira fī ar-risāla

A useful note:

• الوظيفة is pronounced al-waẓīfa
• الشاغرة is pronounced ash-shāghira
• الرسالة is pronounced ar-risāla

That is because:

• ش and ر are sun letters, so the l sound of الـ assimilates
• but و is not a sun letter, so الوظيفة keeps the l sound

Because ش is one of the sun letters.

With sun letters, the ل in الـ is not pronounced; instead, the following consonant is doubled.

So:

• الشاغرة is written with ال
• but pronounced ash-shāghira

The same thing happens in:

• الرسالة → ar-risāla
• الشمس → ash-shams

By contrast, الوظيفة starts with و, which is not a sun letter, so it stays:

• al-waẓīfa

That would make the phrase indefinite instead of definite.

Compare:

• وظيفة شاغرة = an open position / a vacant job
• الوظيفة الشاغرة = the open position / the vacant job

So the version with الـ usually refers to a specific job opening already known from context.

If fully vowelled, the phrase would most likely be:

الوظيفةُ الشاغرةُ في الرسالةِ

Why?

• الوظيفةُ: nominative
• الشاغرةُ: nominative, matching the noun
• الرسالةِ: genitive after في

In everyday Arabic texts, these final short vowels are usually omitted, so you normally see:

الوظيفة الشاغرة في الرسالة

without the case endings written.

No. It is not an iḍāfa.

It is:

• a noun
  • adjective

So:

• الوظيفة = the job
• الشاغرة = the vacant

An iḍāfa would be a possessive-type structure such as:

• باب البيت = the door of the house
• مدير الشركة = the manager of the company

But here, الشاغرة is describing the noun, not possessing it.

So الوظيفة الشاغرة means:

• the vacant job not
• the job of vacancy

The ة is called tāʾ marbūṭa.

It often marks a noun as feminine. That is one reason وظيفة is treated as feminine, which is why the adjective also appears in the feminine form:

• وظيفة شاغرة

In pronunciation:

• in pause, ة usually sounds like -a
• when followed by case endings in fully vocalized Arabic, it can sound like -at / -atu / -ati, depending on grammar

So:

• pause form: waẓīfa
• full grammatical form: waẓīfatu or waẓīfati, depending on case

Yes. رسالة can mean:

• letter
• message
• epistle
• written communication

So في الرسالة could mean:

• in the letter
• in the message

The exact choice depends on context. In Modern Standard Arabic, رسالة is quite flexible.