Game to be in a revolutionary vessel (5)
I believe the answer is:
chess
'game' is the definition.
(chess is a kind of game)
'a revolutionary vessel' is the wordplay.
'a revolutionary' becomes 'che' (Che Guevara).
'vessel' becomes 'SS' (short for steamship).
'che'+'ss'='CHESS'
'to be in' acts as a link.
(Other definitions for chess that I've seen before include "This is played on board" , "Sixteen-men-a-side game" , "In which two bishops will participate" , "Musical - board game" , "Ancient war game whose name may have derived for the Persian for 'king'" .)