Crossword Genius

Creature in reservoir coming from the bottom, thousands mutating (4)

Ross

I believe the answer is:

gnat

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'creature' is the definition.
(gnat is a kind of creature)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for gnat that I've seen before include "Tiny biter" , "Small biting flying insect" , "Small biting fly" , "little irritation" , "insect's" .)

I've seen this clue in The Telegraph.
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