Number otherwise for singer (5)
I believe the answer is:
tenor
'for singer' is the definition.
(tenor is a kind of singer)
'number otherwise' is the wordplay.
'number' becomes 'ten' (ten is a kind of number).
'otherwise' becomes 'or'.
'ten'+'or'='TENOR'
(Other definitions for tenor that I've seen before include "Pavarotti, for example" , "Big bell" , "Carreras, for example" , "General course - male singer" , "Singer; drift" .)