Clothing to boast about (4)
I believe the answer is:
garb
'clothing' is the definition.
(I know that garb is a type of costume)
'boast about' is the wordplay.
'boast' becomes 'brag' (brag is a kind of boast).
'about' says the letters should be written backwards.
'brag' backwards is 'GARB'.
'to' acts as a link.
(Other definitions for garb that I've seen before include "something to wear" , "Style of clothing" , "Dress - clothing" , "Kit" , "gear discovered" .)