Talk about headwear (4)
I believe the answer is:
chat
'talk' is the definition.
(chat can mean to talk)
'about headwear' is the wordplay.
'about' becomes 'c' (circa).
'headwear' becomes 'hat' (hat is a type of headwear).
'c'+'hat'='CHAT'
(Other definitions for chat that I've seen before include "Inconsequential conversation" , "gas" , "A bird; converse" , "Exchange text or voice messages through a computer" , "(13) [FRENCH] Cat" .)