Talk about milliner (7)
I believe the answer is:
chatter
'talk' is the definition.
(chatter is a kind of talk)
'about milliner' is the wordplay.
'about' becomes 'c' (circa abbreviation).
'milliner' becomes 'hatter' (synonyms).
'c'+'hatter'='CHATTER'
(Other definitions for chatter that I've seen before include "Trivial conversation" , "Idle talk" , "Prattle" , "Talk noisily" , "Talk idly" .)