See a letter goes to the agent (4)
I believe the answer is:
espy
'see' is the definition.
(to espy is to catch sight of it)
'a letter goes to the agent' is the wordplay.
'a letter' becomes 'e'.
'goes to' means one lot of letters go next to another.
'the agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Catch a glimpse of" , "Watch" , "Behold" , "Pick out at a distance" , "Observe (arch.)" .)