Nonsense, but good to sing about (5)
I believe the answer is:
hokum
'nonsense' is the definition.
(I've seen this before)
'good to sing about' is the wordplay.
'good' becomes 'ok'.
'to sing' becomes 'hum' (humming is a kind of singing).
'about' is an insertion indicator.
'ok' inserted within 'hum' is 'HOKUM'.
'but' is the link.
(Other definitions for hokum that I've seen before include "Rubbish" , "Sentimental stuff in play or film, nonsense" , "Claptrap" , "Meaninglessness" , "gammon" .)