Talk about headgear (4)
I believe the answer is:
chat
'talk' is the definition.
(thesaurus)
'about headgear' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'headgear' becomes 'hat' (hat is a kind of headgear**).
'c'+'hat'='CHAT'
(Other definitions for chat that I've seen before include "Talk informally; bird" , "Talk amiably - bird" , "tittle-tattle" , "Exchange text or voice messages through a computer" , "Natter" .)