something incidental or added for ornament, symmetry, etc.

Music . a part in a composition designed to serve as background and support for more important parts.

noun

accessory , adjunct , appendage , appurtenance , attachment , attendant , attribute , augmentation , complement , concomitant , enhancement , enrichment , supplement , back , background , backing , back-up , harmony , instrument , part , associate , companion , agreement , chaperon , circumstantiality , collaboration , concomitance , convoy , corollary , guard , melody , obligato , protection , support