Stiff clerical cap (7)
I believe the answer is:
biretta
'stiff clerical cap' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for biretta that I've seen before include "Battier (anag.)" , "Cloth cap" , "religious headwear" , "Clergyman's cap" , "Clerical headgear" .)