Difficult agent holds back drink (7)
I believe the answer is:
stroppy
'difficult' is the definition.
(I've seen this before)
'agent holds back drink' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'holds' indicates putting letters inside.
'back' shows that the letters should be reversed in order.
'drink' becomes 'port' (port is a kind of drink).
'port' written backwards gives 'trop'.
'spy' placed around 'trop' is 'STROPPY'.
(Other definitions for stroppy that I've seen before include "Awkward to handle" , "Unhelpful" , "Ill-tempered" , "Obstreperous" , "like a razor-sharpener?" .)