Learn Turkish: Alması icap eden ilişik belgesi henüz hazır değil. - The clearance document that he is required to get is not ready yet.

In this structure, we use a verbal noun with a possessive suffix followed by the relative clause modifier icap eden (which necessitates/requires). Alması means "his getting". Together, alması icap eden [belge] literally means "[the document] which necessitates his getting", but it translates smoothly to "that he is required to get."

How do we know it's "he" who has to get the document? Where is the pronoun?

The subject is indicated by the possessive suffix on the verbal noun. Al-ma-sı has the third-person singular possessive suffix -sı, meaning "his/her getting". If the sentence meant "that I am required to get", it would be al-ma-m icap eden.

Is icap eden the same as gereken? Could I use alması gereken here instead?

Yes, they are completely interchangeable in this grammar structure. Alması gereken is actually much more common in everyday, modern Turkish. İcap eden means the exact same thing but sounds slightly more formal or bureaucratic, which is why it fits perfectly here when talking about official paperwork.

Is the -si at the end of belgesi saying it is "his" document?