الحافة ملساء.

Yes, it is a complete sentence.

In Arabic, the present-tense verb to be is usually not stated in simple sentences like this. So Arabic often says, literally:

• الحافة ملساء
• the edge smooth

but the natural English meaning is:

• The edge is smooth.

This kind of sentence is called a nominal sentence. Here:

• الحافة = the subject/topic
• ملساء = the predicate/description

If you wanted was, Arabic would use a verb, for example:

• كانت الحافة ملساء = The edge was smooth.

Because الحافة ملساء is a sentence, not a noun phrase.

• الحافة ملساء = The edge is smooth
• الحافة الملساء = the smooth edge

This is a very important Arabic pattern:

• definite noun + indefinite adjective/predicate → usually a sentence
• definite noun + definite adjective → a noun phrase

So:

• حافة ملساء = a smooth edge
• الحافة الملساء = the smooth edge
• الحافة ملساء = The edge is smooth

That difference in definiteness is one of the main clues.

Because الحافة is a feminine noun, and Arabic adjectives agree with the nouns they describe in gender.

So the feminine adjective is:

• ملساء = smooth for a feminine noun

The masculine form is:

• أملس

For example:

• سطح أملس = a smooth surface
• حافة ملساء = a smooth edge

Even though ملساء is the predicate here, it still agrees with الحافة in gender.

Because in Arabic, descriptive adjectives normally come after the noun, not before it.

So Arabic says:

• حافة ملساء literally edge smooth

not something like smooth edge in English order.

The same basic order appears here:

• الحافة ملساء

Also, in this sentence ملساء is not just an adjective inside a phrase; it is the predicate of the whole sentence. But either way, Arabic naturally places this kind of describing word after the noun.

A careful pronunciation is roughly:

• al-ḥāffatu malsāʾu
  or in pause:
• al-ḥāffa malsāʾ

A few details:

• الـ is pronounced al- here, because ح is a moon letter, so the l sound stays.
• ح is a deeper, breathier sound than ordinary English h.
• الحافة has a doubled f sound: ḥāf-fa
• the final ة is usually heard as -a / -ah when you pause
• ملساء ends with ء, so the end has a small glottal stop: mal-sāʾ

So the second word is not just malsaa; it ends with a little catch in the throat: malsāʾ.

That final ة is called taa marbuuTa (ة). It is very common on feminine nouns.

In pause, it is usually pronounced like -a or -ah, which is why you hear:

• الحافة ≈ al-ḥāffa

But when grammar endings are pronounced, or when the word is linked in certain constructions, the t sound comes back. For example, in careful reading:

• الحافّةُ = al-ḥāffatu

So ة is one reason the noun is recognized as feminine.

The fully vowelled form is:

• الحافَّةُ مَلْساءُ

In traditional grammar:

• الحافَّةُ is the mubtadaʾ (subject/topic)
• مَلْساءُ is the khabar (predicate)

Both are in the nominative case, which is why they take nominative endings in full formal vocalization.

In ordinary reading and speech, those short final case vowels are often not pronounced, so you usually just see and hear:

• الحافة ملساء

Because ملساء belongs to a group of words that are often treated as diptotes (ممنوعة من الصرف).

That means they do not take tanween, even when indefinite.

So in full case marking, you get:

• nominative: ملساءُ
• accusative: ملساءَ
• genitive: ملساءَ

not:

• ملساءٌ
• ملساءٍ

A similar common example is:

• حمراءُ = red (feminine)

This is a slightly more advanced point, but it explains why the word is indefinite without showing tanween.