A toff - one who acts for another (5)
I believe the answer is:
agent
'one who acts for another' is the definition.
(I've seen this before)
'a toff' is the wordplay.
'toff' becomes 'gent' (both can mean an upper-class man).
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "Author" , "One's representative" , "Vehicle" , "Business manager" , "Catalyst" .)