You can't. A shifter with 7 clicks is intended for an 8-speed cassette and the cog spacing is much different than for a 6-speed cassette. You need a matching shifter, or a wheel that will take an 8-speed cassette, or you have to use your shifters in friction mode if it has one.

I once bought a used bike with a 6-speed cassette and 7-speed downtube shifters. The original owner couldn't figure out why it wouldn't index properly either which is why I got it cheap.