Crossword Genius

Instrument that may be vital? (5)

Ross

I believe the answer is:

organ

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for organ that I've seen before include "Perhaps one that's stopped" , "Wind instrument found in the body." , "Biological functionary" , "Medium of information - most churches have one" , "Wurlitzer, for example" .)

I've seen this clue in The Telegraph.
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