Square cap worn by Roman Catholic clergy (7)
I believe the answer is:
biretta
'square cap worn by roman catholic clergy' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for biretta that I've seen before include "Cardinal's red cap" , "Stiff clerical cap" , "religious headwear" , "Cloth cap" , "ecclesiastical headgear" .)