Seabird’s opportunity to be heard (4)
I believe the answer is:
tern
'seabird's' is the definition.
(tern is a kind of seabird)
'opportunity to be heard' is the wordplay.
'opportunity' becomes 'turn' (having a turn means having an opportunity).
'to be heard' indicates a 'sounds like' (homophone) clue.
'turn' is a homophone of 'TERN'.
(Other definitions for tern that I've seen before include "flier" , "Small gull with forked tail" , "Winger" , "Gull-like sea bird" , "Seabird, sounds like become sour" .)