It will not be a totally safe combination, but I would guess it would be pretty safe IF the assassins are kept well fed the whole time. Although I've never kept them myself, Assassins (Clea helena) are a fw species belonging to the same Buccinidae family that is frequently seen in the marine side of the hobby where they are used as scavengers to clean up uneaten meaty foods. From what I have seen of assassins, even though they've never been in my tanks, they seem pretty similar to their marine counterparts in terms of both behavior and general anatomy. Buccinid whelks have a proboscis (hard to see; it's kept retracted except when feeding) that allows them to reach through very small openings to feed, so safety is not guaranteed just because another snail is very large or has a trap door. Although not all Buccinids have the same degree of predatory tendency and many are preferentially scavengers, predation is always a risk with Buccinids if they become desperate enough to hunt more actively. When they want to, Buccinid whelks can take out snails much larger than themselves by just eating away at them slowly.
