Map left inside vessel (4)
I believe the answer is:
plan
'map' is the definition.
(I know that map is a more specific form of the action plan)
'left inside vessel' is the wordplay.
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'vessel' becomes 'pan' (vessel or container for cooking).
'l' placed inside 'pan' is 'PLAN'.
(Other definitions for plan that I've seen before include "Project" , "Formulate" , "Intend" , "Blueprint" , "Scheme - map" .)