Extra digit written outside box (2,5)
I believe the answer is:
to spare
'extra' is the definition.
(I've seen this in another clue)
'digit written outside box' is the wordplay.
I cannot quite understand how this works, but
'box' could be 'spar' (spar can mean to box) and 'spar' is found within the answer.
'digit' could be 'toe' (digit on a foot) and 'toe' is present in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for to spare that I've seen is " Left over".)