No one in composition of Wagner is Scandinavian (9)
I believe the answer is:
norwegian
'is scandinavian' is the definition.
(I've seen this before)
'no one in composition of wagner' is the wordplay.
'one' becomes 'i' (Roman numeral**).
'in' means one lot of letters goes inside another.
'composition of' indicates an anagram (I've seen this in other clues).
'wagner' is an anagram of 'rwegan'.
'i' going within 'rwegan' is 'rwegian'.
'no'+'rwegian'='NORWEGIAN'
(Other definitions for norwegian that I've seen before include "From Oslo, perhaps" , "National" , "1 in 21 [OSLO] possibly" , "European" , "Native to, eg, Oslo" .)