Musical by Rice one knocked back as unoriginal (9)
I believe the answer is:
imitative
'unoriginal' is the definition.
('imitative' can be similar in meaning to 'unoriginal')
'musical by rice one knocked back' is the wordplay.
'musical' becomes 'evita' (I've seen this before**).
'by' says to put letters next to each other.
'rice' becomes 'tim' (Sir Tim Rice).
'one' becomes 'i' (Roman numeral).
'knocked back' says the letters should be written in reverse.
'evita'+'tim'+'i'='evitatimi'
'evitatimi' backwards is 'IMITATIVE'.
'as' is the link.
(Other definitions for imitative that I've seen before include "Following a model" , "Apish, emulative" , "is like an ape" , "Mimicking or onomatopoeic" , "pirated" .)