Huh?
I might be wrong, but isn't a clearway a form of airspace?
Let's see how to define it...Hmmm. Maybe "The airspace extending from some point on a departure runway encompassing a 2-3 degree flight path (I'm not sure which) up to an altitude of 400' AGL".
I might be confusing terms with something else, but I do think a clearway has something to do with a certain climb gradient, either on one or two engines to a certain altitude at a certain distance from either the point of lift off or from the departure end of the runway.
A displaced threshold, OTOH, is a paved surface that can be used for taxi and takeoff but not for landing (identified by chevrons pointing you toward where you may land). And a stopway (I think) is something you can roll out on but not use for other purposes.
I'll be interested to hear anybody else's definitions.