Plant animals rejected (4)
I believe the answer is:
reed
'plant' is the definition.
(reed is a kind of plant)
'animals rejected' is the wordplay.
'animals' becomes 'deer' (type of animals).
'rejected' is a reversal indicator.
'deer' backwards is 'REED'.
(Other definitions for reed that I've seen before include "Blade of tall marsh grass" , "Instrument; tall grass" , "Marsh plant with straight hollow stem" , "Music pipe" , "Tall stiff marsh or water grass" .)