Bird overseas on prey (6)
I believe the answer is:
osprey
'bird' is the definition.
(osprey is a kind of bird)
'overseas on prey' is the wordplay.
'overseas' becomes 'os' (abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'os'+'prey'='OSPREY'
(Other definitions for osprey that I've seen before include "Spy roe (anag)" , "Large bird that fishes inland waters" , "Large seabird" , "Fish-eating hawk" , "Flier" .)