Horse — one Queen ordered (6)
I believe the answer is:
equine
'horse' is the definition.
(I've seen this before)
'one queen ordered' is the wordplay.
'one' becomes 'i' (roman numeral).
'ordered' indicates anagramming the letters.
'i'+'queen'='iqueen'
'iqueen' is an anagram of 'EQUINE'.
(Other definitions for equine that I've seen before include "Resembling a horse" , "Horsy" , "Horse-like" , "Relating to a horse" , "Relating to horses" .)