Oscar, with regret, rebuffed French bread (4)
I believe the answer is:
euro
'french bread' is the definition.
(I've seen this in another clue)
'oscar with regret rebuffed' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' says to put letters next to each other.
'regret' becomes 'rue' (to rue something is to regret it).
'rebuffed' is a reversal indicator.
'rue' in reverse letter order is 'eur'.
'o' put after 'eur' is 'EURO'.
(Other definitions for euro that I've seen before include "Cash" , "Currency of Greece" , "International currency unit" , "EU money" , "Currency of Finland" .)