Lower the French ship (4)
I believe the answer is:
less
'lower' is the definition.
(synonyms)
'the french ship' is the wordplay.
'the french' becomes 'le' ('the' in French).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'le'+'ss'='LESS'
(Other definitions for less that I've seen before include "Fewer in number" , "See 12" , "Wanting" , "A smaller amount" , "Fewer; minus" .)