Bird with a forked tail (4)
I believe the answer is:
tern
'bird with a forked tail' 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 "Seagull" , "Gull-like seabird" , "Fork-tailed relative of the gull" , "Small gull with forked tail" , "Fork-tailed seabird" .)