Old mare wanders grand estate (7)

I believe the answer is:

earldom

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'grand estate' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'old mare wanders' is the wordplay.
'wanders' indicates anagramming the letters.
'old'+'mare'='oldmare'
'oldmare' is an anagram of 'EARLDOM'.

Can you help me to learn more?

(Other definitions for earldom that I've seen before include "Dignity" , "Domain of a mid-ranking peer" , "Noble's lands or rank" , "Noble rank" , "Aristocratic domain above a viscountcy but below a marquessate" .)

I've seen this clue in The Independent.

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