Nonsense, but good in sound (5)
I believe the answer is:
hokum
'nonsense' is the definition.
(hokum can mean nonsense or rubbish)
'good in sound' is the wordplay.
'good' becomes 'ok'.
'in' is an insertion indicator.
'sound' becomes 'hum' (humming is a kind of sounding).
'ok' going within 'hum' is 'HOKUM'.
'but' acts as a link.
(Other definitions for hokum that I've seen before include "Be extremely good (at activity)" , "Meaninglessness" , "Hogwash" , "Claptrap" , "gammon" .)