Wild ox found in ancient city states (4)
I believe the answer is:
urus
'wild ox' is the definition.
(urus is a kind of wild ox)
'ancient city states' is the wordplay.
'ancient city' becomes 'ur' (ancient Sumerian city state).
'states' becomes 'US' (abbreviation for the United States).
'ur'+'us'='URUS'
'found in' acts as a link.
(Other definitions for urus that I've seen before include "Aurochs" , "old beef supplier" , "Animal hunted to extinction" , "bovine, now extinct" .)