Sink back and talk (3)
I believe the answer is:
gas
'talk' is the definition.
(gas can mean to chat)
'sink back' is the wordplay.
'sink' becomes 'sag' (sagging is a kind of sinking).
'back' is a reversal indicator.
'sag' written backwards gives 'GAS'.
'and' acts as a link.
(Other definitions for gas that I've seen before include "Gossip" , "Something exciting" , "THIRD PART OF 23" , "eg Helium" , "State of matter not solid or liquid" .)