Rush animal from below (4)
I believe the answer is:
reed
'rush' is the definition.
(plants found by water)
'animal from below' is the wordplay.
'animal' becomes 'deer' (deer is a type of animal).
'from below' is a reversal indicator (in a down clue, letters go up).
'deer' reversed gives 'REED'.
(Other definitions for reed that I've seen before include "Tall grass at water's edge" , "Tall grass plant that grows in water" , "Stalk" , "Riverside grass" , "Tall stiff marsh or water grass" .)