Half-tamed horse (6)
I believe the answer is:
bronco
'half-tamed horse' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bronco that I've seen before include "An unbroken mustang, bucking perhaps" , "Wild horse of the West" , "Wild bucking horse" , "Half-tamed horse used in rodeos" , "Wild horse of W. US" .)