I spy a toff (5)
I believe the answer is:
agent
'i spy' is the definition.
'agent' can be an answer for 'spy' (secret agent). I'm not sure about the 'i' bit.
'a toff' is the wordplay.
'toff' becomes 'gent' (both can mean an upper-class man).
'a'+'gent'='AGENT'
Can you help me to learn more?
(Other definitions for agent that I've seen before include "One's representative" , "factor in Scotland" , "He or she acts for actors" , "envoy" , "Instrument" .)