Concert, in a sense (6)

I believe the answer is:

unison

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'concert' is the definition.
(to work in concert is to work in unison)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for unison that I've seen before include "Sameness in pitch" , "Concordance" , "Concord; a trade union" , "Complete agreement, harmony" , "Type of singing" .)

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