Marshy area includes river plant (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'marshy area includes river' is the wordplay.
'marshy area' becomes 'fen' (I've seen this before).
'includes' is an insertion indicator.
'river' becomes 'r'.
'fen' enclosing 'r' is 'FERN'.
(Other definitions for fern that I've seen before include "Girl - bracken" , "Flowerless plant with feathery green leaves, common on mountains" , "Plant producing spores" , "Feathery frondy plant" , "It has feathery or leafy fronds" .)