Vessel circling lake in secret plan (4)
I believe the answer is:
plot
'secret plan' is the definition.
(synonyms)
'vessel circling lake' is the wordplay.
'vessel' becomes 'pot' (type of container).
'circling' indicates putting letters inside (circling can mean going around or containing).
'lake' becomes 'L' (geographical abbreviation).
'pot' going around 'l' is 'PLOT'.
'in' acts as a link.
(Other definitions for plot that I've seen before include "Novel story - secret plan" , "Graph; piece of ground" , "Contrive" , "Concoct schemes" , "bed" .)