We installed tiles one tile doesn't have mortar under it in one area for about 1/2-3/4 of an inch under it. Can we mix up some more mortar then squeeze it under the tile to prevent it from breaking due to lack of support?

Is this along an exposed edge or what?

If you can get the mortar in, do so. Otherwise, remove the tile and start over. How big is the tile and why is the hole so deep? That's a very thick mortar base. .

Yes, it is along an exposed edge. I can push a business card into the opening about 3/4-1inch and the length is maybe 2-21/2 inches.

Mix a small batch of thinset out of a bag, use dry mix stuff not from a can. Mix it creamy like peanut butter and trowel it in or even use a pastry bag, they make them for mortar too.

Thanks. I tried out your suggestion and on checking it appears that the mortar is under the tile now. Thanks