See the key goes to the agent (4)
I believe the answer is:
espy
'see' is the definition.
(to espy is to catch sight of it)
'key goes to the agent' is the wordplay.
'key' becomes 'E' (a musical key).
'goes to' means one lot of letters go next to another.
'the agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
'the' is the link.
(Other definitions for espy that I've seen before include "Spot; observe" , "Make out at a distance" , "Perceive" , "Spot, notice" , "Observe (arch.)" .)