Boast about what one is wearing (4)
I believe the answer is:
garb
'what one is wearing' is the definition.
(garb means clothing)
'boast about' is the wordplay.
'boast' becomes 'brag' (brag is a kind of boast).
'about' says the letters should be written backwards.
'brag' in reverse letter order is 'GARB'.
(Other definitions for garb that I've seen before include "Fashion of dress" , "Attire, clothing" , "something to wear" , "Distinctive dress of a group" , "gear discovered" .)