A number given to someone to sing (4)
I believe the answer is:
alto
'someone to sing' is the definition.
(I've seen this before)
'a number given to' is the wordplay.
'number' becomes 'l' (L is an example).
'given' is a charade indicator (letters next to each other).
'a'+'l'+'to'='ALTO'
(Other definitions for alto that I've seen before include "The highest adult male singing voice" , "Singer's hidden inside chemical toilet" , "Low female singing voice" , "Woman's singing voice" , "Kind of singing voice, male or female" .)