Why would Clarity use .FM for its domain?

Because other, more common domains were not available. FM is not exclusive to music, and is some times used to mean "for me"

Phone consultations aren't that far removed from a call-in radio show.

And getting clear, actionable advice depends on tuning in to the right station or channel. You can think of the various experts as numbers on the FM dial.

On both those levels, Clarity.fm is apt. Quite a clever brand name, I'd say. Both relevant and memorable.

I'm sure they are losing some traffic clarity.com - which redirects to clarityoss.com.

Someone from the Clarity Team would be best suited to answer this, but I think the association with "FM" -> Radio -> Talkshow is probably why they picked this TLD. Don't think they are based in Micronesia ;-)

I've also got a FM domain since it matches my initials, how could I resist.