Passage in a musical score not to be omitted (9)

I believe the answer is:

obbligato

(Other definitions for obbligato that I've seen before include "Essential part of a music score" , "Instrumental part integral to a piece of music - I go to blab (anag)" , "Instrumental part not to be omitted" , "Has to be done" , "Compulsory (music)" .)

I've seen this clue in the Sydney Morning Herald.

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