Say, Fagin and Artful appearing in not so successful musical (3,10)
I believe the answer is:
les miserables
'successful musical' is the definition.
'say fagin and artful appearing in not' is the wordplay.
'say fagin' becomes 'miser' (Fagin from Oliver Twist**).
'and' means one lot of letters go next to another.
'artful' becomes 'able'.
'appearing in' indicates putting letters inside.
'not' becomes 'less' (I've seen this in other clues).
'miser'+'able'='miserable'
'miserable' going within 'less' is 'LES MISERABLES'.
'so' is the link.
(Other definitions for les miserables that I've seen before include "show" , "Musical from Victor Hugo" , "Hugo-based musical" , "Well-known musical" , "Victor Hugo novel, Valjean its hero" .)