Liqueur not at all unknown to the French (5)
I believe the answer is:
noyau
'liqueur' is the definition.
(French liqueur)
'not at all unknown to the french' is the wordplay.
'not at all' becomes 'no'.
'unknown' becomes 'y' (algebra).
'to the french' becomes 'au' ('to the' in French).
'no'+'y'+'au'='NOYAU'
(Other definitions for noyau that I've seen before include "Brandy flavoured with fruit kernels; an you (anag.)" , "French liqueur" , "Liqueur of brandy and peach kernels" , "Liquor made of brandy and nuts" .)