هذا الجانب نظيف.

Because Arabic often leaves out the present-tense verb to be.

So هذا الجانب نظيف literally looks like this side clean, but it naturally means this side is clean.

This is a normal nominal sentence in Arabic. In the present tense, Arabic usually does not say is / am / are. If you wanted the past tense, you would use a verb such as كان.

For example:

• هذا الجانب نظيف = This side is clean
• كان هذا الجانب نظيفًا = This side was clean

That is just how standard Arabic works. After a demonstrative like هذا or هذه, the noun is usually still marked as definite with الـ.

So Arabic says:

• هذا الجانب
  literally something like this the-side

Even though that feels redundant to an English speaker, it is completely normal in Arabic.

Other examples:

• هذا الكتاب = this book
• هذه السيارة = this car

Because جانب is a masculine singular noun.

In Arabic, demonstratives must match the noun in gender and number:

• هذا = this for masculine singular
• هذه = this for feminine singular

So:

• هذا الجانب = correct
• هذه الجانب = incorrect in MSA

Because here نظيف is the predicate of the sentence, not an adjective directly attached to the noun.

Compare these two:

• هذا الجانب نظيف = This side is clean
  Here نظيف tells you something about this side.

• هذا الجانب النظيف = This clean side
  Here النظيف directly describes الجانب as part of the noun phrase.

So in your sentence, نظيف is indefinite because it is the predicate in a nominal sentence.

It is still an adjective in meaning, but in this sentence it functions as a predicate adjective.

In other words:

• lexical category: adjective
• sentence role: predicate (خبر)

So نظيف means clean, and it is the part that says something about هذا الجانب.

In fully vocalized formal Arabic, it would be:

هذا الجانبُ نظيفٌ

Here is why:

• الجانبُ is nominative
• نظيفٌ is also nominative

This is because the sentence is a nominal sentence with:

• هذا الجانب as the topic/subject-like part
• نظيف as the predicate

In normal print and in most speech, these endings are usually not written or pronounced clearly, so you commonly just see:

هذا الجانب نظيف

It is pronounced hādhā.

A few helpful points:

• the first ā is long
• ذ sounds like the th in this
• the final ā is also long

So هذا is roughly: haa-thaa, with the th of this, not thin

The full sentence can be approximated as: hādhā al-jānib naẓīf

No. جانب can mean several related things depending on context, including:

• side
• aspect
• part
• side of an issue

So هذا الجانب نظيف could mean:

• This side is clean in a physical sense
• This aspect is clean in a more abstract sense

Context tells you which meaning is intended.

In Modern Standard Arabic, the normal order is:

هذا الجانب

So for MSA, هذا الجانب نظيف is the standard form.

Some Arabic dialects do allow noun + demonstrative patterns like الجانب هذا, but that is not the usual MSA structure. If you are learning formal Arabic, stick with هذا الجانب.

You would say:

هذا الجانب النظيف

That is different from your original sentence:

• هذا الجانب نظيف = This side is clean
• هذا الجانب النظيف = This clean side

The difference is important:

• predicate adjective: نظيف
• attributive adjective modifying the noun directly: النظيف

When an adjective directly describes a definite noun in Arabic, it also becomes definite with الـ.