Revising his set subject (6)
I believe the answer is:
thesis
'subject' is the definition.
(I've seen this before)
'revising his set' is the wordplay.
'revising' is an anagram indicator.
'his'+'set'='hisset'
'hisset' anagrammed gives 'THESIS'.
(Other definitions for thesis that I've seen before include "writing following research" , "Theme for an essay" , "Academic essay" , "Dissertation based on research" , "Lengthy written essay for university degree" .)