not safe at all. the point of a tubular is that it is physically glued to the rim. a clincher is more or less held in place by a hook shape of the rim interacting with a corresponding bead shape on the tire forced into place by air pressure.

can it be done in a pinch.... sure... would I feel safe on it. nope.

there's the oddball tufo tubular clincher as well, which is essentially a tubular tire with the corresponding clincher bead as part of the base strip.

In any case... tubular tire on tubular rim. clincher tire on clincher rim, don't every try to mix and match.