Simple pond plant (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
alga
(Other definitions for alga that I've seen before include "Simple plant, plankton or seaweed say" , "nuisance on the beach?" , "Gala class of seaweed" , "Possibly tangle" , "Simple, non-flowering, aquatic plant" .)