Claptrap, but good in sound (5)
I believe the answer is:
hokum
'claptrap' is the definition.
(I've seen this before)
'good in sound' is the wordplay.
'good' becomes 'ok'.
'in' indicates putting letters inside.
'sound' becomes 'hum' (humming is a kind of sounding).
'ok' placed within 'hum' is 'HOKUM'.
'but' is the link.
(Other definitions for hokum that I've seen before include "Claptrap" , "Bunk" , "Nonsense (colloq.)" , "Sentimental stuff in play or film, nonsense" , "Meaninglessness" .)