There isn’t really a herring one (4)
I believe the answer is:
pond
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for pond that I've seen before include "Pool of still water" , "Possible site for weed" , "Small body of water" , "Small area of still water" , "Small lake" .)