Waning moon, possibly seen on the surface of the water (8)
I believe the answer is:
meniscus
'water' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for meniscus that I've seen before include "Curve on liquid surface" , "Disk of cartilage at a joint" , "Curve on fluid surface in a tube" , "Curved upper surface of liquid in a tube" , "Overfilled glass shows it!" .)