Scarcity of boxes containing pictures, with 100 missing (10)

I believe the answer is:

sparseness

'scarcity' is the definition.
(thesaurus)

'boxes containing pictures with 100 missing' is the wordplay.
I cannot really understand how this works, but
'boxes' could be 'spars' (sparring is a kind of boxing) and 'spars' is found in the answer.
This may be the basis of clue (or it may be nonsense).

'of' is the link.

(Another definition for sparseness that I've seen is " Scantiness".)

I've seen this clue in The Times.

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