Black attire unsuitable with cap (7)
I believe the answer is:
biretta
'cap' is the definition.
(biretta is a kind of cap)
'black attire unsuitable' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil hardness).
'unsuitable' indicates anagramming the letters (letters not in suitable order).
'attire' with letters rearranged gives 'iretta'.
'b'+'iretta'='BIRETTA'
'with' acts as a link.
(Other definitions for biretta that I've seen before include "Cleric takes this on himself" , "that covers a Catholic priest" , "Square cap worn by Roman Catholic clergy" , "Cloth cap" , "Stiff clerical cap" .)