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' says the letters should be written backwards (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" .)