Instrument which sounds unbelievable (4)

I believe the answer is:

lyre

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

I can't explain the rest of the clue.

(Other definitions for lyre that I've seen before include "Harp used by ancient Greeks" , "Ancient Greece harp" , "Ancient Greek stringed instrument" , "Instrument played by Orpheus" , "Plucked instrument" .)

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

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