Plant found in marsh bounding river (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'marsh bounding river' is the wordplay.
'marsh' becomes 'fen' ('fen' can be a synonym of 'marsh').
'bounding' indicates putting letters inside.
'river' becomes 'r'.
'fen' going around 'r' is 'FERN'.
'found in' is the link.
(Other definitions for fern that I've seen before include "Plant with featherlike leaves" , "Fronded plant" , "Non-flowering plant" , "Feathery frond plant" , "Often shady plant" .)