Migratory seabird (4)
I believe the answer is:
tern
'migratory seabird' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tern that I've seen before include "Fork-tailed seabird" , "Fork-tailed relative of the gull" , "Seabird related to the gull" , "Pay rent for seabird" , "Sea bird, a small gull" .)