I bought a picture frame as diorama base and can't remove the glass.Will air dry clay adhere to glass with pva glue on glass first or will the clay adhere to glass on its own.Thanks.

When it's still wet it will stick ,but after it has completely dried you can most likely pull it off from the glass,
Use some contact glue and glue it back on the glass when the clay has fully cured.

Smash the glass?....

...or you could fix some thin ply/mdf onto the glass maybe?

Never tried air-drying clay on glass because it cracks and splits (the clay, that is) when drying. But I have used ready-mix patching cement directly on a glass surface successfully.