obbligato

obligato puhekieltä Designating part of a musical piece which is subordinate to the main melody but still essential; designating a crucial instrumental part. (defdate)
puhekieltä An obbligato section; a prominent countermelody, often written to be played or sung above the principal theme (in a higher pitch range). (defdate)
1953, (w), The Long Goodbye, Penguin 2010, p. 281:

The noise of the traffic outside the building on the boulevard made an unmusical obbligato to my thinking.

percorso obbligato - fixed course