Soprano needs backing of beatbox (when given space) (4)
I believe the answer is:
'given space' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'soprano needs backing of beatbox' is the wordplay.
'soprano' becomes 's' (musical abbreviation).
'needs' means one lot of letters go next to another.
'backing of' says the letters should be written backwards.
'beatbox' becomes 'rap' (I am not sure about this - if you are sure you should believe this answer much more).
'rap' backwards is 'par'.
'when' is the link.
(Other definitions for spar that I've seen before include "Length of wood" , "quarrel" , "Do up (bird for oven)" , "Sticks" , "Practice boxing with wooden beam" .)