Notice eastern agent (4)
I believe the answer is:
espy
'notice' is the definition.
(I've seen this before)
'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 "Perceive" , "Catch sight of (poetic)" , "Make out at a distance" , "Spot; observe" , "Behold" .)