Pond organisms (5)
I believe the answer is:
algae
'pond organisms' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for algae that I've seen before include "Type of seaweed" , "They grow near water and don't have ordinary roots or leaves" , "Aquatic growths" , "Water plants seen in a gale" , "'Simple plants, such as seaweed (5)'" .)