Extra box with digit on the outside (2,5)
I believe the answer is:
Here is my best explanation:
'extra' is the definition.
'box with digit on the outside' is the subsidiary indication.
'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' going inside 'toe' is 'tospare'.
(Another definition for to spare that I've seen is "Left over".)