One being given sustenance by another (8)
I believe the answer is:
cannibal
'one being' is the definition.
(cannibal is a kind of being)
I can't explain the remainder of the clue.
(Other definitions for cannibal that I've seen before include "One eating own species" , "One eats human flesh" , "Person eating human flesh" , "One with odd eating habits" , "Animal that eats its own kind" .)