Is the kingdom protista a true clade?

No, the 'Kingdom' Protista is not a clade or monophyletic group at all, most likely. It could be that it should be divided into about 60 separate kingdoms. 'Protista' was used as a catch-all for unclassifyable groups. This surely does not reflect reality and work is under way to properly classify the algae and protista that swarm in this improper group called Protista.