Creature in reservoir coming from the bottom, thousands mutating (4)
I believe the answer is:
gnat
'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" .)