Spy is a polite fellow (5)
I believe the answer is:
agent
'spy' is the definition.
(I've seen this before)
'a polite fellow' is the wordplay.
'polite fellow' becomes 'gent' (synonyms. I am not sure about the 'polite' bit.).
'a'+'gent'='AGENT'
'is' is the link.
Can you help me to learn more?
(Other definitions for agent that I've seen before include "He or she acts for actors" , "Vehicle" , "envoy" , "Spy; service provider" , "that may be secret?" .)