European river engulfing slender shrub (8)
I believe the answer is:
oleander
'shrub' is the definition.
(thesaurus)
'european river engulfing slender' is the wordplay.
'european river' becomes 'oder' (I've seen this before).
'engulfing' means one lot of letters goes inside another.
'slender' becomes 'lean' ('lean' can be similar in meaning to 'slender').
'oder' placed around 'lean' is 'OLEANDER'.
(Other definitions for oleander that I've seen before include "Evergreen flowering shrub from olden era" , "bloomer" , "No dealer in evergreen shrub" , "'Adorn eel, badly, with this shrub (8)'" , "Red alone (anag.)" .)