Fork-tailed seabird (4)
I believe the answer is:
tern
'fork-tailed seabird' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tern that I've seen before include "Seabird related to the gull" , "Pay rent for seabird" , "Small slender gull" , "Long-winged aquatic bird" , "Fork-tailed relative of the gull" .)