Crossword Genius

Extra box with digit on the outside (2,5)

Ross

I believe the answer is:

to spare

'extra' is the definition.
(thesaurus)

'box with digit on the outside' is the wordplay.
'box with' becomes 'spar' (sparring is a kind of boxing. I am not sure about the 'with' bit.**).
'digit' becomes 'toe' (toe is a kind of digit).
'on the outside' indicates putting letters inside.
'spar' inserted inside 'toe' is 'TO SPARE'.

(Another definition for to spare that I've seen is " Left over".)

I've seen this clue in The Guardian.
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