He sits composing dissertation (6)
I believe the answer is:
thesis
'dissertation' is the definition.
('thesis' can be a synonym of 'dissertation')
'he sits composing' is the wordplay.
'composing' is an anagram indicator.
'he'+'sits'='hesits'
'hesits' with letters rearranged gives 'THESIS'.
(Other definitions for thesis that I've seen before include "Research essay written to obtain university degree" , "student's work" , "theory" , "research paper" , "Analytical essay written for university degree" .)