Very round coin placed on back of bureau quickly (6)
I believe the answer is:
subito
'quickly' is the definition.
(thesaurus)
'very round coin placed on back of bureau' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'round' means one lot of letters goes inside another.
'coin' becomes 'bit' (term for various historical coins).
'placed on' is a charade indicator (letters next to each other).
'back of' says to take the final letters.
The last letter of 'bureau' is 'u'.
'bit' put after 'u' is 'ubit'.
'so' placed around 'ubit' is 'SUBITO'.
(Other definitions for subito that I've seen before include "(In music) suddenly" , "Immediately" , "Suddenly musical" .)