Cruel temperament, variable (6)
I believe the answer is:
bloody
'cruel' is the definition.
(thesaurus)
'temperament variable' is the wordplay.
'temperament' becomes 'blood' (blood is a kind of temperament).
'variable' becomes 'y' (I've seen this in other clues).
'blood'+'y'='BLOODY'
(Other definitions for bloody that I've seen before include "Cruel, murderous" , "Gory" , ""____ but unbowed"" .)