Nonsense it's alright to sing about (5)
I believe the answer is:
hokum
'nonsense' is the definition.
(thesaurus)
'alright to sing about' is the wordplay.
'alright' becomes 'ok' ('OK' can be a synonym of 'alright').
'to sing' becomes 'hum' (humming is a kind of singing).
'about' is an insertion indicator.
'ok' going into 'hum' is 'HOKUM'.
'it's' is the link.
(Other definitions for hokum that I've seen before include "Rubbish" , "Hogwash" , "Bunk" , "Meaninglessness" , "Claptrap" .)