Ground elder on a shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(thesaurus)
'ground elder on a' is the wordplay.
'ground' indicates anagramming the letters (grind the letters ).
'elder'+'on'+'a'='elderona'
'elderona' anagrammed gives 'OLEANDER'.
(Other definitions for oleander that I've seen before include "Poisonous evergreen shrub with white, pink or red flowers" , "'Adorn eel, badly, with this shrub (8)'" , "Evergreen flowering shrub from olden era" , "Flowering plant" , "Flowering shrub from olden era, oddly" .)