French booze — idiot thus knocked back bottles (6)
I believe the answer is:
cassis
'french booze' is the definition.
(as in creme de cassis liqueur)
'idiot thus knocked back bottles' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'thus' becomes 'sic' ('thus' in Latin).
'knocked back' is a reversal indicator.
'bottles' means one lot of letters goes inside another (bottle can mean to contain).
'sic' back-to-front is 'cis'.
'ass' inserted within 'cis' is 'CASSIS'.
(Other definitions for cassis that I've seen before include "Blackcurrant cordial" , "Syrupy blackcurrant liqueur" , "Liqueur made with blackcurrants" , "syrup drink" , "French blackcurrant mixer" .)