See key goes to agent (4)
I believe the answer is:
espy
'see' is the definition.
(thesaurus)
'key goes to agent' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'goes to' is a charade indicator (letters next to each other).
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "See" , "Observe (arch.)" , "Catch sight of (poetic)" , "Spot at a distance" , "Pick out at a distance" .)