Talk, then hang up (3)
I believe the answer is:
gas
'talk' is the definition.
(gas can mean to chat)
'hang up' is the wordplay.
'hang' becomes 'sag' (to sag is to droop or hang down).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'sag' backwards is 'GAS'.
'then' is the link.
(Other definitions for gas that I've seen before include "Wind" , "See 2" , "idle chit-chat" , "Eg, nitrogen" , "Nitrogen maybe" .)