Old queen keeping horse and wild ass (6)
I believe the answer is:
onager
'wild ass' is the definition.
(onager is a kind of wild ass)
'old queen keeping horse' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'keeping' means one lot of letters goes inside another.
'horse' becomes 'nag' (nag is a kind of horse).
'o'+'er'='oer'
'oer' enclosing 'nag' is 'ONAGER'.
'and' is the link.
(Other definitions for onager that I've seen before include "donkey" , "The ass is a strange orange" , "Creature" , "Orange Asiatic ass" , "The ass might be an odd orange colour" .)