Lie alongside instrument backing (4)

I believe the answer is:

abut

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'lie alongside' is the definition.
(I've seen this in another clue)

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for abut that I've seen before include "Adjoin -- 19 (rev)" , "Cease (from activity)" , "Be next to -- share a boundary" , "Be next door to" , "Neighbour" .)

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

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