Agent drinking strong wine up becoming argumentative (7)
I believe the answer is:
stroppy
'argumentative' is the definition.
(I've seen this before)
'agent drinking strong wine up' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'strong wine' becomes 'port' (strong type of wine).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'port' reversed gives 'trop'.
'spy' going around 'trop' is 'STROPPY'.
'becoming' is the link.
(Other definitions for stroppy that I've seen before include "Obstreperous" , "Awkward to handle" , "like a razor-sharpener?" , "Bad-tempered, argumentative" , "Unhelpful" .)