Force of evil once let loose (8)
I believe the answer is:
violence
'force' is the definition.
(force can mean violence or aggression)
'evil once let loose' is the wordplay.
'let loose' indicates an anagram.
'evil'+'once'='evilonce'
'evilonce' with letters rearranged gives 'VIOLENCE'.
'of' acts as a link.
(Other definitions for violence that I've seen before include "Ferocity" , "Unjustifiable force" , "Harmful behaviour" , "Destructive force" , "Act of aggression, physical force" .)