Rosette's a frivolity? Don't listen! (4,1,4,2)

I believe the answer is:

cock a deaf un

'don't listen' is the definition.

'rosette's a frivolity?' is the wordplay.
'rosette' becomes 'cockade' (I've seen this in another clue).
'frivolity?' becomes 'fun' (fun is a kind of frivolity).
'cockade'+'a'+'fun'='COCK A DEAF UN'

I've seen this clue in The Guardian.

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