Bird that is astonishing but heartless (8)
I believe the answer is:
starling
'bird' is the definition.
(starling is a kind of bird)
'is astonishing but heartless' is the wordplay.
'is astonishing' becomes 'startling' (I've seen this before).
'but heartless' suggests removing the centre.
'startling' with its middle taken out is 'STARLING'.
'that' is the link.
(Other definitions for starling that I've seen before include "Gregarious songbird" , "Slant rig for the strange bird" , "Dark, glossy bird" , "Noisy bird with speckled feathers - somewhat stellar" , "Gregarious bird" .)