On the ship the French were reduced (4)
I believe the answer is:
less
'reduced' is the definition.
(a reduced amount)
'on the ship the french' is the wordplay.
'on' means one lot of letters go next to another.
'the ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'the french' becomes 'le' ('the' in French).
'ss' after 'le' is 'LESS'.
'were' is the link.
(Other definitions for less that I've seen before include "So-called St James" , "inferior" , "A smaller amount" , "See 12" , "Comparatively little" .)