Clamp causes Stationary which again causes the unit with that effect to be unable to move by all means. However Stationary seems to be bugged so that it doesnt have an effect beyond the effect itself. Tested this with Giant Crustocean.
Yep it doesnt seem to do anything, I have done very little testing, but my hypothesis now is that clamp gives stationary for 1 turn, and should give 2 turns. edit: just had a game where it worked as it should.
From what I was able to find is the stationary condition is removed when any champion disengages from the affected champion not just the one who gave it the condition. This could be a result of many different issues, but we won't know for sure until Gedden is able to take a look at what is happening in the code. Thanks for bringing this to our attention.
In the course of testing I had it too that no champion disengaged and yet the unit wouldnt be really stationary. Iirc the stationary condition showed on the opposing champion but there was simply no effect to it.