Flyer to circle we hear (4)
I believe the answer is:
tern
'flyer' is the definition.
'circle we hear' is the wordplay.
'circle' becomes 'turn'.
'we hear' shows a homophone (sound like).
'turn' is a homophone of 'TERN'.
'to' is the link.
(Other definitions for tern that I've seen before include "Fork-tailed seabird" , "Seabird, sounds like become sour" , "Seagull" , "Small gull with forked tail" , "Small seabird with forked tail" .)