Belter is rotten singer (6)
I believe the answer is:
treble
'singer' is the definition.
(I know that treble is a type of singer)
'belter is rotten' is the wordplay.
'is rotten' indicates an anagram.
'belter' anagrammed gives 'TREBLE'.
(Other definitions for treble that I've seen before include "Soprano - multiply by three" , "Feat of winning three major trophies" , "Threefold; boy's voice" , "high frequency" , "Triple - high-pitched" .)