What’s brother’s haircut? Not flipping certain! (7)
I believe the answer is:
tonsure
'what's brother's haircut?' is the definition.
(tonsure means a monk's partly shaved head)
'not flipping certain' is the wordplay.
'flipping' is a reversal indicator.
'certain' becomes 'sure' (synonyms).
'not' backwards is 'ton'.
'ton'+'sure'='TONSURE'
(Other definitions for tonsure that I've seen before include "Bald bit" , "Shaven area on monk's head" , "Monk's haircut" , "Part of a monk's head left bare by shaving" , "Shaved crown of a monk's head" .)