Spy a toff (5)
I believe the answer is:
agent
'spy' is the definition.
(thesaurus)
'a toff' is the wordplay.
'toff' becomes 'gent' (I've seen this in other clues).
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "Delegate" , "One may be working" , "Vehicle" , "Author" , "Catalyst" .)