Cool fool, don't ya agree? (5)
I believe the answer is:
innit
'don't ya agree?' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'cool fool' is the wordplay.
'cool' becomes 'in' ('in' can mean fashionable or cool).
'fool' becomes 'nit' (both can mean a stupid person).
'in'+'nit'='INNIT'
Can you help me to learn more?
(Other definitions for innit that I've seen before include "do you agree?" , "Common question" , "Isn't it (sl.)" , "Mere spoken punctuation" , "that produces stress for some" .)