European river contains thin, poisonous plant (8)
I believe the answer is:
oleander
'poisonous plant' is the definition.
(I know that oleander is a type of poisonous plant)
'european river contains thin' is the wordplay.
'european river' becomes 'oder' (River Oder in central Europe).
'contains' means one lot of letters goes inside another.
'thin' becomes 'lean' (associated in meaning).
'oder' enclosing 'lean' is 'OLEANDER'.
(Other definitions for oleander that I've seen before include "Flowering shrub from olden era, oddly" , "'Adorn eel, badly, with this shrub (8)'" , "Mediterranean shrub" , "Poisonous shrub, also called rosebay" , "No leader, oddly, for the flowering shrub" .)