Stagnant, perhaps, but it's full of life! (4)
I believe the answer is:
pond
'life' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pond that I've seen before include "Himalayan country" , "Small area of still water" , "Small body of water" , "Possible site for weed" , "Small lake" .)