He'll sing a number as an alternative (5)
I believe the answer is:
tenor
'he'll sing a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'number as an alternative' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'as an alternative' becomes 'or' (I've seen this in other clues).
'ten'+'or'='TENOR'
Can you help me to learn more?
(Other definitions for tenor that I've seen before include "Higher male voice" , "Vocal part" , "Placido Domingo, say" , "thread" , "Meaning - male singing voice" .)