One-celled creature found in water (6)
I believe the answer is:
amoeba
'one-celled creature found in water' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for amoeba that I've seen before include "parasitic type" , "One-celled creature found in water" , "Protozoan of ever-changing shape" , "Tiny one-celled animal in water" , "Unicellular living thing" .)