Upset - not certain of a haircut (7)
I believe the answer is:
tonsure
'a haircut' is the definition.
(I've seen this before)
'upset not certain' is the wordplay.
'upset' says the letters should be written in reverse.
'certain' becomes 'sure' (similar in meaning).
'not' backwards is 'ton'.
'ton'+'sure'='TONSURE'
'of' is the link.
(Other definitions for tonsure that I've seen before include "Monk's haircut" , "Shaven part of monk's head" , "Shaved crown of a monk's head" , "Partial shaving of head" , "Result of shaving" .)