Cain -- he'd turned into a beast (7)
I believe the answer is:
echidna
'a beast' is the definition.
(echidna is a kind of beast)
'cain he'd turned' is the wordplay.
'turned' indicates an anagram.
'cain'+'hed'='cainhed'
'cainhed' anagrammed gives 'ECHIDNA'.
'into' acts as a link.
(Other definitions for echidna that I've seen before include "Australian creature" , "Oz native" , "Spiny Aus. mammal" , "Insectivorous mammal" , "Creature with spines" .)