Talk, then hang up (3)
I believe the answer is:
gas
'talk' is the definition.
(I've seen this before)
'hang up' is the wordplay.
'hang' becomes 'sag'.
'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" .)