Cantankerous agent drinking wine sent back (7)
I believe the answer is:
stroppy
'cantankerous' is the definition.
'agent drinking wine sent back' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'drinking' indicates putting letters inside.
'wine' becomes 'port' (port is a kind of wine**).
'sent back' is a reversal indicator.
'port' back-to-front is 'trop'.
'spy' enclosing 'trop' is 'STROPPY'.
(Other definitions for stroppy that I've seen before include "Obstreperous" , "Bolshie" , "Ill-tempered" , "Bad-tempered, argumentative" , "Awkward to handle" .)