Plot prepared by criminals, leading to robbery at sea (10)
I believe the answer is:
conspiracy
'plot' is the definition.
(I know that plot can be written as conspiracy)
'criminals leading to robbery at sea' is the wordplay.
'criminals' becomes 'cons' (short for convicts).
'leading to' means one lot of letters go next to another (some letters go before or lead others).
'robbery at sea' becomes 'piracy' (what pirates do).
'cons'+'piracy'='CONSPIRACY'
'prepared by' acts as a link.
(Other definitions for conspiracy that I've seen before include "Secret plan" , "This is intriguing" , "Plot" , "intrigue" , "Joint action undertaken secretly" .)