Conspiracy of fifty on board vessel (4)
I believe the answer is:
plot
'conspiracy' is the definition.
(a conspiracy is a secret plot)
'fifty on board vessel' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'on board' means one lot of letters goes inside another (inserted letters are taken on board).
'vessel' becomes 'pot' (type of container).
'l' put into 'pot' is 'PLOT'.
'of' acts as a link.
(Other definitions for plot that I've seen before include "are intriguing" , "Novel story - secret plan" , "Draw (graph)" , "Secret plan; graph" , "Secret scheme or plan" .)