Fashionable fool, would you not agree? (5)
I believe the answer is:
innit
'would you not agree?' is the definition.
(informal way of saying 'isn't it?')
'fashionable fool' is the wordplay.
'fashionable' becomes 'in' (similar in meaning).
'fool' becomes 'nit' (both can mean a stupid person).
'in'+'nit'='INNIT'
(Other definitions for innit that I've seen before include "word of emphasis" , "tag question" , "Isn't it (sl.)" , "Mere spoken punctuation" , "do you agree?" .)