Half a score otherwise for singer (5)
I believe the answer is:
tenor
'singer' is the definition.
(tenor is a kind of singer)
'half a score otherwise' is the wordplay.
'half a score' becomes 'ten' (I've seen this before).
'otherwise' becomes 'or'.
'ten'+'or'='TENOR'
'for' is the link.
(Other definitions for tenor that I've seen before include "Pavarotti, for example" , "Vocal range" , "Eg, Caruso" , "thread" , "Pitch range" .)