Catch sight of eastern agent (4)
I believe the answer is:
espy
'catch sight of' is the definition.
(espying is a kind of catching sight of)
'eastern agent' is the wordplay.
'eastern' becomes 'e'.
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Discern" , "Watch" , "Catch sight of (poetic)" , "Pick out at a distance" , "Spot at a distance" .)