Instrument a violinist can arrange to sit in with? (4)

I believe the answer is:

viol

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'instrument' is the definition.
(viol is a kind of instrument)

I don't understand the rest of the clue.

(Other definitions for viol that I've seen before include "Early string instrument" , "Old consort instrument" , "Early fiddle" , "Early bowed stringed instrument" , "Double bass relative" .)

I've seen this clue in the Evening Standard.

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