A drink for the agent containing cold rum (7)
I believe the answer is:
scrumpy
'a drink' is the definition.
(I have seen 'Strong drink' mean 'scrumpy' so perhaps 'drink' could also mean 'scrumpy')
'agent containing cold rum' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'containing' indicates putting letters inside.
'cold' becomes 'c' (eg on taps).
'c'+'rum'='crum'
'spy' enclosing 'crum' is 'SCRUMPY'.
'for the' is the link.
(Other definitions for scrumpy that I've seen before include "Rough dry cider" , "Rough strong cider" , "alcohol" , "rough drink" , "Strong English cider" .)