Catch glimpse of European agent (4)
I believe the answer is:
espy
'catch glimpse of' 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 "Catch a glimpse of" , "Spot; observe" , "Perceive" , "Spot, notice" , "Spot at a distance" .)