One doing wrong upsetting songsters? Rare? Not entirely (12)
I believe the answer is:
transgressor
'one doing wrong' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'upsetting songsters? rare? not entirely' is the wordplay.
'upsetting' indicates an anagram.
'not entirely' means to remove the last letter (most but not all of the word).
'songsters'+'rare'='songstersrare'
'songstersrare' with its final letter taken off is 'songstersrar'.
'songstersrar' anagrammed gives 'TRANSGRESSOR'.
Can you help me to learn more?
(Other definitions for transgressor that I've seen before include "One who oversteps the limit" , "One who oversteps a mark" , "Offender; sinner" , "Lawbreaker - grass snorter (anag)" .)