It's good when you talk! (3)
I believe the answer is:
gas
'talk' is the definition.
(gas can mean to chat)
'it's good when' is the wordplay.
'it's good' becomes 'g' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'g'+'as'='GAS'
'you' acts as a link.
(Other definitions for gas that I've seen before include "Possibly noble" , "Chatter vapidly" , "idle chit-chat" , "Hydrogen or methane, say" , "Hydrogen, for example" .)