Prisoner's robbery plot (10)
I believe the answer is:
conspiracy
'plot' is the definition.
(I know that conspiracy is a type of plot)
'prisoner's robbery' is the wordplay.
'prisoner's' becomes 'con's' (abbreviation for 'convict').
'robbery' becomes 'piracy' (piracy is a kind of robbery).
'cons'+'piracy'='CONSPIRACY'
(Other definitions for conspiracy that I've seen before include "This is intriguing" , "Secret plan" , "Joint action undertaken secretly" , "that involves plotting" , "Plot" .)