Demon niece turned out to be an entertainer (10)
I believe the answer is:
comedienne
'entertainer' is the definition.
(comedienne is a kind of entertainer)
'demon niece turned out' is the wordplay.
'turned out' indicates an anagram (turn out can mean to organise).
'demon'+'niece'='demonniece'
'demonniece' anagrammed gives 'COMEDIENNE'.
'to be an' is the link.
(Other definitions for comedienne that I've seen before include "stand-up entertainer?" , "Rivers perhaps" , "Female comedy actor" , "Funny female performer" , "Female humorist" .)