Pond covering (5)
I believe the answer is:
algae
'pond covering' 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 "Scum" , "They grow near water and don't have ordinary roots or leaves" , "Seaweeds generally" , "Type of seaweed" , "Seaweeds etc" .)