Box no longer wanted with side missing (4)
I believe the answer is:
spar
'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" .)