One acting on behalf of another (5)
'one acting on behalf of another' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for agent that I've seen before include "Negotiator" , "Double-zero-seven, for example" , "Delegate" , "factor in Scotland" , "Spy; representative" .)
(Other definitions for proxy that I've seen before include "One voting on behalf of another" , "Stand in" , "Delegated vote" , "Authorised substitute" , "Authority to represent another" .)