Returning musical includes Dutch princess as extra (8)
I believe the answer is:
additive
'extra' is the definition.
'returning musical includes dutch princess' is the wordplay.
'returning' is a reversal indicator.
'musical' becomes 'evita' (I've seen this before**).
'includes' means one lot of letters goes inside another.
'dutch' becomes 'd' (common abbreviation).
'princess' becomes 'di'.
'evita' back-to-front is 'ative'.
'd'+'di'='ddi'
'ative' going around 'ddi' is 'ADDITIVE'.
'as' is the link.
(Other definitions for additive that I've seen before include "Tie David up with added substance" , "ingredient" , "Flavour enhancer" , "Colouring maybe" , "Something put into food" .)