The French vessel carries a smaller amount (4)
I believe the answer is:
less
'a smaller amount' is the definition.
(I've seen this before)
'the french vessel carries' is the wordplay.
'the french' becomes 'le' ('the' in French).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'carries' is a charade indicator (letters next to each other) (in a down clue, some letters go beneath others).
'le'+'ss'='LESS'
(Other definitions for less that I've seen before include "See 12" , "Smaller in amout or degree" , "more, as they say" , "Minus; not so much" , "So-called St James" .)