Instrument that may be vital? (5)
I believe the answer is:
organ
'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" .)