Listening out after releasing one small bird (8)
I believe the answer is:
nestling
'small bird' is the definition.
(a bird that's still in the nest)
'listening out after releasing one' is the wordplay.
I cannot really see how this works, but
'one' could be 'li' (I've seen this in other clues) and 'li' is found in the answer.
This explanation may well be incorrect...
(Other definitions for nestling that I've seen before include "Sitting snugly" , "getting under duvet?" , "Chick" , "Settling in comfortably, against someone perhaps" , "Little bird" .)