Seagull (4)
I believe the answer is:
tern
'seagull' 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 "Long-winged aquatic bird" , "Gull-like seabird" , "Seabird, sounds like become sour" , "Small gull with forked tail" , "Pay rent for seabird" .)