Instrument sounds unbelievable (4)
I believe the answer is:
lyre
'instrument' is the definition.
(lyre 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 lyre that I've seen before include "Early harp" , "Music-maker" , "Plucked instrument" , "Ancient Greek harp" , "Harp used by ancient Greeks" .)