Strange attire topped by black cap (7)
I believe the answer is:
biretta
'cap' is the definition.
(biretta is a kind of cap)
'strange attire topped by black' is the wordplay.
'strange' indicates an anagram.
'topped by' is a charade indicator (letters next to each other).
'black' becomes 'b' (as in pencil classifications**).
'attire' anagrammed gives 'iretta'.
'iretta' after 'b' is 'BIRETTA'.
(Other definitions for biretta that I've seen before include "Battier (anag.)" , "religious headwear" , "Cleric takes this on himself" , "Cardinal's red cap" , "ecclesiastical headgear" .)