Yes, I thought this was strange too as the Chambers entries under cassino itself and under casino mention only the card game. However, I'm assuming MOCASSIN is the correct solution.
I am surprised at that ontarioice - the New International Websters Comprehensive Dictionary has for cassino - see casion (def 3). Defintion 3 for casino is for the game and not for the room or building. Perhaps you would quote what exactly your Websters says under cassino?
This is a very old Websters. First meaning is: A town in Italy Northwest of Naples etc. With the second meaning: Variant of Casino.
I also double checked online at The Free Dictionary site and it gives same meanings.
I suspect the Chambers problem is that the semi-colon is in the wrong place...ie seemingly tying the cassino version to only the game. Certainly the double-s variant is included in The Oxford English Dictionary as an alternative to 'casino' meaning the gaming-house and it completes the 'mocassin' clue perfectly fitting all surrounding answers.