Wild horse (7)
'wild horse' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mustang that I've seen before include "Trotter" , "creature" , "Wild horse of the West" , "Small, half-wild horse of the U.S." , "Small, wild US horse" .)
(Other definitions for brumby that I've seen before include "Wild horse in Australia" , "One Australian breaks in" , "H [HORSE]" .)