Even Clio Cook produced act of aggression (8)
I believe the answer is:
violence
'act of aggression' is the definition.
(I've seen this before)
'even clio cook' is the wordplay.
'cook' indicates anagramming the letters.
'even'+'clio'='evenclio'
'evenclio' anagrammed gives 'VIOLENCE'.
'produced' is the link.
(Other definitions for violence that I've seen before include "Destructive force" , "Ferocity" , "Act of aggression, physical force" , "Btutality" , "Harmful physical force" .)