Aquatic bird on three-masted schooner (4)
I believe the answer is:
tern
This clue is a double definition.
'aquatic bird' is the first definition.
(tern is a kind of aquatic bird)
'three-masted schooner' is the second definition.
(I've seen this before)
'on' acts as a link.
(Other definitions for tern that I've seen before include "Long-winged aquatic bird" , "Seabird, sounds like become sour" , "Winger" , "Pay rent for seabird" , "Small gull with forked tail" .)