The scheme puts fifty in the vessel (4)
I believe the answer is:
plan
'the scheme' is the definition.
(synonyms)
'fifty in the vessel' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'in the' indicates putting letters inside.
'vessel' becomes 'pan' (vessel or container for cooking).
'l' put into 'pan' is 'PLAN'.
'puts' is the link.
(Other definitions for plan that I've seen before include "Detailed proposal" , "Design; map" , "Scheme to achieve something" , "Scale drawing; scheme" , "Method laid out for accomplishing something" .)