One shows little authenticity in sound of instrument (4)
I believe the answer is:
liar
'one shows little authenticity' is the definition.
(someone inauthentic or dishonest)
'sound of instrument' is the wordplay.
'sound of' shows a homophone (sound like).
'instrument' becomes 'lyre' (lyre is a kind of instrument).
'lyre' is a homophone of 'LIAR'.
'in' acts as a link.
(Other definitions for liar that I've seen before include "Worse than a thief" , "Rail at a prevaricator" , "Pinocchio, at times" , "Perjuror" , "Person telling stories" .)