Popular idiot's common supplementary question, right? (5)
I believe the answer is:
innit
'common supplementary question right?' is the definition.
'popular idiot's' is the wordplay.
'popular' becomes 'in'.
'idiot' becomes 'nit' (I've seen this before).
'in'+'nit'='INNIT'
(Other definitions for innit that I've seen before include "Isn't it (sl.)" , "tag question" , "that produces stress for some" , "Mere spoken punctuation" , "word of emphasis" .)