Notice European agent (4)
I believe the answer is:
espy
'notice' is the definition.
(I've seen this in another clue)
'european agent' is the wordplay.
'european' becomes 'E' (abbreviation e.g. EU).
'agent' becomes 'spy' (spy is a kind of agent).
'e'+'spy'='ESPY'
(Other definitions for espy that I've seen before include "Perceive" , "Spot, notice" , "Make out at a distance" , "Observe (arch.)" , "Watch" .)