Alder - one exotic shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(thesaurus)
'alder one exotic' is the wordplay.
'exotic' indicates an anagram.
'alder'+'one'='alderone'
'alderone' with letters rearranged gives 'OLEANDER'.
(Other definitions for oleander that I've seen before include "Flowering shrub from olden era, oddly" , "Poisonous evergreen shrub with white, pink or red flowers" , "Poisonous shrub, also called rosebay" , "poisonous plant" , "tree" .)