"Clincher" rims/tires (whether tubeless or not) means for standard tires ie with a bead as opposed to "tubular" tires/rims which are mainly used for elite road & cyclo-cross racing.
I'd think about just getting a road bike for now--already set up for fast riding & they don't break on dirt/gravel, just a bit bumpier. Gravel or 'cross bike with extra wheels, new crankset & brake lever/shifter can get expensive & still a compromise esp w/gearing. So why bother when you already have a MTB for off-road?