Talk about Roundhead costume (4)
I believe the answer is:
garb
'costume' is the definition.
(I know that costume is a kind of garb)
'talk about roundhead' is the wordplay.
'talk' becomes 'gab' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'roundhead' becomes 'r' (head letter of 'round').
'gab' going around 'r' is 'GARB'.
(Other definitions for garb that I've seen before include "Style of clothing" , "Distinctive dress of a group" , "Dress - clothing" , "Kit" , "Covering" .)