Weed garden's first bed raised in grass (7)
I believe the answer is:
ragwort
'weed' is the definition.
(ragwort is a kind of weed)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for ragwort that I've seen before include "Yellow-flowered weed" , "Yellow flower, poisonous to cattle" , "13 across toxic to livestock" , "Plant toxic to livestock" , "Widespread obnoxious weed" .)