See a key agent (4)
I believe the answer is:
espy
'see' is the definition.
(thesaurus)
'a key agent' is the wordplay.
'a key' becomes 'e' (I've seen this in other clues).
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Detect" , "Discern" , "Behold" , "Observe (arch.)" , "See" .)