Hat could be attire designed by Balenciaga originally (7)
I believe the answer is:
biretta
'hat' is the definition.
(I know that biretta is a hat)
'attire designed by balenciaga originally' is the wordplay.
'designed' is an anagram indicator (designed with the letters).
'by' says to put letters next to each other.
'originally' indicates taking the first letters.
The first letter of 'balenciaga' is 'b'.
'attire' with letters rearranged gives 'iretta'.
'iretta' after 'b' is 'BIRETTA'.
'could be' acts as a link.
(Other definitions for biretta that I've seen before include "Cloth cap" , "Roman Catholic priest's cap" , "Clerical headgear" , "Stiff cap worn by Roman Catholic clergy" , "Battier (anag.)" .)