Tiny shape-shifter
I believe the answer is:
amoeba
'tiny shape-shifter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amoeba that I've seen before include "Protozoan life form" , "Tiny organism consisting of single changing cell" , "Basic life form" , "Tiny one-celled animal in water" , "shape-shifter" .)