Flatter setting for third rate flower (6,3)

I believe the answer is:

butter cup

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'rate flower' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'flatter setting for third' is the wordplay.
'flatter' becomes 'butterup' (buttering up is a kind of flattering).
'setting for' indicates putting letters inside (inserted letters are set inside others).
'third' becomes 'c' (I am not sure about this - if you are sure you should believe this answer much more).
'butterup' placed around 'c' is 'BUTTER CUP'.

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I've seen this clue in The Guardian.

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