can I use a 10 speed brake lever shifter If I have a 7 gear cassette I'm restoring an old Myatta 1000 and want to use a brake lever shifter that I have. It is a 10 speed shifter and I have a 7 gear cassette. Can i use this?
No.

The brake lever has to be matched to the cable pull of the brake (short pull/long pull); V-brakes need different cable pull than caliper brakes, for example. Else, you'll have not enough power to stop or you'll lock up the wheel instantly.

The number of speeds on the shifter needs to match the number of speeds on the cassette (*). The shifter also has to be compatible with the derailleur. 

Also, note that road and mountain shift+brake levers have different clamp diameters since the handlebars have different diameters, so you need to use road shifters+brake levers on a road bar, and mountain shifters+brake levers on a flat bar. 

(*) There are a few special cases where you can violate this. 7 speed cassette with 10 speed shifter is not one of them. 



In this case (assuming Miyata 1000; a drop bar road bike), I'd suggest finding a compatible brake lever (which will be easy), and using friction shifters (downtube/bar end). Or find a 7 speed indexed shifter which is compatible with the derailleur at hand (if the rear derailleur is Shimano, theres now a 7 speed brifter in the Tourney range, as well as one made by microshift; you can easily find 7 speed compatible index/friction switchable downtube or bar end shifters for Shimano as well).