What might delay black grouse breeding, returning wasted (8)
I believe the answer is:
skeletal
'wasted' is the definition.
(both can mean extremely thin)
I don't understand the rest of the clue.
(Other definitions for skeletal that I've seen before include "pinched" , "Bony, gaunt" , "Pulchritude" , "wasted" , "Comprising just the bare bones" .)