Ruminant comes up for grass (4)
I believe the answer is:
reed
'for grass' is the definition.
(I've seen this before)
'ruminant comes up' is the wordplay.
'ruminant' becomes 'deer' (deer is a kind of ruminant).
'comes up' shows that the letters should be reversed in order.
'deer' backwards is 'REED'.
(Other definitions for reed that I've seen before include "Blade of tall marsh grass" , "Grassy plant of wet areas" , "Type of musical instrument" , "Thin strip that vibrates to produce a note" , "oboe part" .)