Talk a lot, then hang up (3)
I believe the answer is:
gas
'talk a lot' is the definition.
(gas can mean to talk at length)
'hang up' is the wordplay.
'hang' becomes 'sag' (I've seen this in another clue).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'sag' written backwards gives 'GAS'.
'then' is the link.
(Other definitions for gas that I've seen before include "State of matter not solid or liquid" , "Eg, oxygen" , "could be embarrassing for American" , "that may quickly be stepped on" , "THIRD PART OF 23" .)