Horse one Queen maltreated (6)
I believe the answer is:
equine
'horse' is the definition.
(equine means a horse or related animal)
'one queen maltreated' is the wordplay.
'one' becomes 'i' (Roman numeral).
'maltreated' indicates anagramming the letters.
'i'+'queen'='iqueen'
'iqueen' with letters rearranged gives 'EQUINE'.
(Other definitions for equine that I've seen before include "like Black Beauty" , "Horse-like" , "Horse or donkey, perhaps" , "Like a horse" , "like knight?" .)