Should be heard during a string quartet (5)

I believe the answer is:

ought

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'should be' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for ought that I've seen before include "Strange thing" , "Anything at all (archaic)" , "outdated anything at all?" , "Be logically necessary" , "Tough (anag)" .)

I've seen this clue in the Evening Standard.

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