Shaving the head is in fashion, certainly (7)
I believe the answer is:
tonsure
'shaving the head' is the definition.
'in fashion certainly' is the wordplay.
'in fashion' becomes 'ton' (fashionable style or distinction).
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'ton'+'sure'='TONSURE'
'is' is the link.
(Other definitions for tonsure that I've seen before include "Shaven spot on monk's head" , "Part of a monk's head left bare by shaving" , "Monk's haircut" , "Shaven area on monk's head" , "Shearing ?" .)