Mere bunk, but good to sing about (5)
I believe the answer is:
hokum
'mere bunk' is the definition.
'hokum' can be an answer for 'bunk' (I've seen this before). I'm not sure about the 'mere' bit.
'good to sing about' is the wordplay.
'good' becomes 'ok'.
'to sing' becomes 'hum' (humming is a kind of singing).
'about' indicates putting letters inside.
'ok' inserted inside 'hum' is 'HOKUM'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for hokum that I've seen before include "Be extremely good (at activity)" , "Claptrap" , "Rubbish" , "Sentimental stuff in play or film, nonsense" , "gammon" .)