Grass certain ruminants brought back (4)
I believe the answer is:
reed
'grass' is the definition.
(type of grass plant)
'ruminants brought back' is the wordplay.
'ruminants' becomes 'deer' (deer are a type of ruminant).
'brought back' shows that the letters should be reversed in order.
'deer' reversed gives 'REED'.
'certain' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for reed that I've seen before include "Grass of marshes" , "Rush" , "Blade of tall marsh grass" , "Tall grass plant that grows in water" , "Coarse grass in the organ pipe" .)