Animals return to grass (4)
I believe the answer is:
reed
'to grass' is the definition.
(I've seen this before)
'animals return' is the wordplay.
'animals' becomes 'deer' (type of animals).
'return' is a reversal indicator.
'deer' written backwards gives 'REED'.
(Other definitions for reed that I've seen before include "Carol" , "blown in the wind" , "Tall grass at water's edge" , "Tall grass plant that grows in water" , "Coarse grass in the organ pipe" .)