Old bank maybe planted with a shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(thesaurus)
'old bank maybe planted with a' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bank maybe' becomes 'lender'.
'planted with' means one lot of letters goes inside another.
'lender' going around 'a' is 'leander'.
'o'+'leander'='OLEANDER'
(Other definitions for oleander that I've seen before include "Flowering plant" , "Poisonous evergreen shrub with white, pink or red flowers" , "'Adorn eel, badly, with this shrub (8)'" , "Reloaned for evergreen shrub with bright flowers" , "tree" .)