A witness taking no part in the proceedings (9)

I believe the answer is:

bystander

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'a witness' is the definition.
(I know that bystander is a type of witness)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for bystander that I've seen before include "Bad sentry may be an onlooker" , "Non-partcipating spectator" , "Non-participant spectator" , "Watcher of occurrence" , "Onlooker (often innocent)" .)

I've seen this clue in The Globe and Mail and The Guardian.

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