Eager to cause carnage, shortly bid to relax (12)
I believe the answer is:
bloodthirsty
'eager to cause carnage' is the definition.
(bloodthirsty means having a desire to kill)
'shortly bid to relax' is the wordplay.
'relax' is an anagram indicator (I've seen 'relaxes' mean this).
'shortly'+'bid'+'to'='shortlybidto'
'shortlybidto' anagrammed gives 'BLOODTHIRSTY'.
(Other definitions for bloodthirsty that I've seen before include "Eager to kill and maim" , "wanting vital liquid assets kept in banks?" , "Craving violence and slaughter" , "like Attila the Hun?" , "Desiring carnage" .)