A hundred certain to have monk's hairstyle (7)
I believe the answer is:
tonsure
'monk's hairstyle' is the definition.
(tonsure means a monk's shaven head)
'a hundred certain' is the wordplay.
'a hundred' becomes 'ton' (I've seen this before).
'certain' becomes 'sure' (similar in meaning).
'ton'+'sure'='TONSURE'
'to have' acts as a link.
(Other definitions for tonsure that I've seen before include "Haircut for a monk" , "Close-shaved hairstyle" , "Monk's bald spot" , "Shaven spot on monk's head" , "Monk's hairdo" .)