Spare property? (8)

I believe the answer is:

leanness

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'spare property?' is the definition.
'leanness' can be an answer for 'property?' (leanness is a kind of property). I'm not sure about the 'spare' bit.

'spare property?' is the wordplay.
'spare' becomes 'lean' (similar in meaning).
'property?' becomes 'ness' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lean'+'ness'='LEANNESS'

Can you help me to learn more?

(Other definitions for leanness that I've seen before include "Lack of fat" , "thin" , "Thinness" .)

I've seen this clue in The Globe and Mail.

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