Plant in marshy area round river (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'marshy area round river' is the wordplay.
'marshy area' becomes 'fen' (I've seen this before).
'round' means one lot of letters goes inside another.
'river' becomes 'r'.
'fen' enclosing 'r' is 'FERN'.
'in' acts as a link.
(Other definitions for fern that I've seen before include "Feathery-leaved plant" , "Plant such as bracken" , "Flowerless plant with feathery green leaves, common on mountains" , "It has feathery or leafy fronds" , "Fronded plant" .)