Very good opera ultimately that inspires American composer (5)
I believe the answer is:
sousa
'composer' is the definition.
(I know that Sousa is a composer)
'very good opera ultimately that inspires american' is the wordplay.
'very good' becomes 'so' (as in the adjectival form 'just so!').
'ultimately' suggests the final letters.
'that inspires' is an insertion indicator (inspire means to breathe in).
'american' becomes 'US' (abbreviation for United States).
The final letter of 'opera' is 'a'.
'so'+'a'='soa'
'soa' going around 'us' is 'SOUSA'.
(Other definitions for sousa that I've seen before include "John Philip ---, US bandmaster" , "American composer known for marches" , "John Philip ..., composer of military marches" , "March man?" , "John Philip -, composer of famous marches (The Stars and Stripes for Ever)" .)