Talk, and sink back (3)
I believe the answer is:
gas
'talk' is the definition.
(thesaurus)
'sink back' is the wordplay.
'sink' becomes 'sag' (sagging is a kind of sinking).
'back' says the letters should be written backwards.
'sag' reversed gives 'GAS'.
'and' acts as a link.
(Other definitions for gas that I've seen before include "could be embarrassing for American" , "Fumes, vapour" , "Hydrogen possibly" , "Hydrogen or methane, say" , "Something delightful" .)