Rank with an old mare, perhaps? (7)
I believe the answer is:
earldom
'rank' is the definition.
(aristocratic rank)
'with an old mare perhaps?' is the wordplay.
'with' means one lot of letters go next to another.
'perhaps?' indicates an anagram (I've seen 'perhaps' mean this).
'old'+'mare' is 'oldmare'.
'oldmare' anagrammed gives 'EARLDOM'.
(Other definitions for earldom that I've seen before include "Noble rank" , "Nobleman's estate" , "Units" , "Lands of a nobleman" , "noble area of country" .)