Half-broken north Colorado mustang? (6)
I believe the answer is:
bronco
'mustang?' is the definition.
(bronco is a kind of mustang)
'half-broken north colorado' is the wordplay.
'half' suggests halving the letters.
'north' becomes 'N' (synonyms).
'colorado' becomes 'co'.
'broken' cut in half is 'bro'.
'bro'+'n'+'co'='BRONCO'
(Other definitions for bronco that I've seen before include "An unbroken mustang, bucking perhaps" , "Untamed mustang" , "Wild horse of the West" , "Lively horse" , "American wild horse" .)