Grass that upsets ruminants (4)
I believe the answer is:
reed
'grass' is the definition.
(type of grass plant)
'upsets ruminants' is the wordplay.
'upsets' says the letters should be written backwards.
'ruminants' becomes 'deer' (deer are a type of ruminant).
'deer' backwards is 'REED'.
'that' is the link.
(Other definitions for reed that I've seen before include "blown in the wind" , "Tall grass that grows in swamps" , "Music pipe" , "Instrument; tall grass" , "Tall water-loving grass" .)