Fork-tailed flier (4)
I believe the answer is:
tern
'fork-tailed flier' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tern that I've seen before include "Winger" , "Gull-like seabird" , "Fork-tailed seabird" , "Seabird related to the gull" , "Three-masted schooner" .)