Conspiracy to put fifty in cooking-vessel (4)
I believe the answer is:
plot
'conspiracy' is the definition.
(I've seen this in other clues)
'fifty in cooking-vessel' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'in' is an insertion indicator.
'cooking vessel' becomes 'pot' (I've seen this before).
'l' going inside 'pot' is 'PLOT'.
'to put' acts as a link.
(Other definitions for plot that I've seen before include "Chart - area" , "Small piece of ground" , "Vegetable garden - scheme" , "Gunpowder, for one" , "Area of ground; storyline" .)