Protozoan of the phylum Rhizopoda able to change shape because of the movements of cell processes (6)
I believe the answer is:
amoeba
(Other definitions for amoeba that I've seen before include "Freshwater or marine protozoa" , "Basic life form" , "Microscopic organism consisting of a single cell changing constantly" , "O, Mam, be a one-celled animal" , "One cell" .)