Spy on a man (5)
I believe the answer is:
agent
'spy' is the definition.
(secret agent)
'on a man' is the wordplay.
'on' says to put letters next to each other.
'man' becomes 'gent' (gent is a kind of man).
'a' next to 'gent' is 'AGENT'.
(Other definitions for agent that I've seen before include "Spy; representative" , "One who acts for another" , "One representing others" , "that may be secret?" , "Traveller" .)