Singer, possibly a belter (6)
I believe the answer is:
treble
'singer' is the definition.
(I've seen this before)
'possibly a belter' is the wordplay.
'possibly' indicates an anagram.
'belter' with letters rearranged gives 'TREBLE'.
(Other definitions for treble that I've seen before include "High-frequency (radio control knob)" , "High voice; tripartite" , "boy in church choir" , "Make or become three times as much" , "High-pitched or soprano, instrument or boy's voice" .)