From the Godfather movie:
Clemenza: Don Corleone, You once said that the day would come Tessio and me could form out own family. Till today I would never think about it. I must ask your permission.
Don: Michael is now the head of the family. If he gives you permission then you have my blessing.
Michael: After we make the move to Nevada, you can break of from the Corleone family and go on your own.
Clemenza: How long will it be?
Michael: 6 months.
Tessio: Forgive me, Godfather. But with you're gone me and Pete will come under Barzini's thumb sooner or later.
Clemenza: I hate that goddamn Barzini. In six motnths' time there will be nothing left to build on.
I can't find more or less sensical meaning of the emphasized sentence. What he was going to build on? What could it mean in that context?