Plant by river in marshland (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'river in marshland' is the wordplay.
'river' becomes 'r'.
'in' indicates putting letters inside.
'marshland' becomes 'fen' (I've seen this before).
'r' put into 'fen' is 'FERN'.
'by' is the link.
(Other definitions for fern that I've seen before include "Tracheophyte plant" , "A frond is its leaf" , "Leafy plant" , "Non-flowering plant" , "Feathery-leaved plant" .)