Spot a key agent (4)
I believe the answer is:
espy
'spot' is the definition.
(I know that spot can be written as espy)
'a key agent' is the wordplay.
'a key' becomes 'E' (a musical key).
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Watch" , "Observe (arch.)" , "See" , "Perceive" , "Catch a glimpse of" .)