A fellow spy (5)
I believe the answer is:
agent
'spy' is the definition.
(secret agent)
'a fellow' is the wordplay.
'fellow' becomes 'gent' (synonyms).
'a'+'gent'='AGENT'
(Other definitions for agent that I've seen before include "One who acts for another" , "factor in Scotland" , "Representative - means" , "Vehicle" , "Sales rep" .)