Bribe the French -- that's a mistake (6)
I believe the answer is:
bungle
'that's a mistake' is the definition.
(bungle is a kind of mistake)
'bribe the french' is the wordplay.
'bribe' becomes 'bung' (I've seen this before).
'the french' becomes 'le' ('the' in French).
'bung'+'le'='BUNGLE'
(Other definitions for bungle that I've seen before include "[BEAR]" , "Do something carelessly or badly" , "(Make an) embarrassing mistake" , "Blunder" , "Clumsy mistake" .)