Algy, we hear, is pretty green (5)
I believe the answer is:
algae
'algy' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I can't see?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for algae that I've seen before include "Primitive organisms" , "Aquatic growths" , "Simple organisms, e.g. seaweeds" , "They grow near water and don't have ordinary roots or leaves" , "Water plants with no true stems or leaves" .)