Bird's stunning, losing heart (8)
I believe the answer is:
starling
'bird's' is the definition.
(starling is a kind of bird)
'stunning losing heart' is the wordplay.
'stunning' becomes 'startling' (startling is a kind of stunning).
'losing heart' means to remove the middle letters.
'startling' with its centre removed is 'STARLING'.
(Other definitions for starling that I've seen before include "Stellar bird with blackish speckled feathers" , "List Gran awkwardly for the bird" , "Passerine bird" , "Bird from heavenly body and fish" , "flier" .)