Crossword Genius

Box no longer wanted with side missing (4)

Ross

I believe the answer is:

spar

I'm a little stuck... Click here to teach me more about this clue!

'box' is the definition.
(sparring is a kind of boxing)

'no longer wanted with side missing' is the wordplay.
I cannot quite see how this works, but
'side' could be 'r' (abbreviation for right) and 'r' is present in the answer.
The remaining letters 'spa' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for spar that I've seen before include "Raps with strong pole" , "Box (gently) - fight (verbally)" , "Raps with large beam" , "Shadow-box" , "Box for practice" .)

I've seen this clue in The Telegraph.
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