UAS flying
I was taught to fly on a UAS. When we went to the aircraft a member of ground crew would come out with you and stand on the wing tip (to remove chocks if it was windy, marshall if needed, etc).
We would shout "clear prop", and which the ground crew member would take a look around, especially behind you, where you can't see, and then answer with "prop clear". Until he replied, you wouldn't hit the starter button.
Following on from this teaching I have always called "clear prop" before starting the engine. I take a good look around first, and know noone is near the prop, but I think of it more as meaning "I'm about to start the engine, so if you're standing directly behind me and don't realise this you're about to get a sudden rush of wind towards you".
But, as I have said in a couple of other threads, I still do my PPL flying at an RAF airfield, and, from what I've seen, everyone else calls "clear prop" as well, and the instructors teach the students to do so.