He sits to compose academic essay (6)
I believe the answer is:
thesis
'academic essay' is the definition.
(I've seen this before)
'he sits to compose' is the wordplay.
'to compose' indicates anagramming the letters.
'he'+'sits'='hesits'
'hesits' anagrammed gives 'THESIS'.
(Other definitions for thesis that I've seen before include "writing following research" , "Student dissertation" , "Dissertation presented for a doctorate" , "He sits (anag) - treatise" , "A lengthy essay, perhaps for degree" .)