You must accept mark for subject (5)
I believe the answer is:
theme
'subject' is the definition.
('theme' can be a synonym of 'subject')
'you must accept mark' is the wordplay.
'you' becomes 'thee' (I've seen this in other clues).
'must accept' means one lot of letters goes inside another.
'mark' becomes 'm'.
'thee' placed around 'm' is 'THEME'.
'for' acts as a link.
(Other definitions for theme that I've seen before include "Subject matter, motif" , "Topic; melody" , "sort of tune" , "issue" , "Recurrent unifying idea" .)