Clare collets were specifically for Clare milling cutters. They were unthreaded in the area where Clarkson cutters are, but they had a special tongue instead. The idea was that you inserted the cutter into an appropriately-sized collet, twisted it a few degrees to engage the tongue with the collet and then tightened the closing nut.
You can sometimes get Clare collets second-hand, but otherwise the collets are pretty well useless. They'll probably grip a Clarkson or plain cutter reasonably well, but the grip won't guarantee it being pulled out.
We used them back in the 1970s in the apprentice school!