Plant in river amid wetland (4)
I believe the answer is:
fern
'plant' is the definition.
(I know that fern is a type of nonflowering plant)
'river amid wetland' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'amid' is an insertion indicator.
'wetland' becomes 'fen' (I've seen this in another clue).
'r' going within 'fen' is 'FERN'.
'in' is the link.
(Other definitions for fern that I've seen before include "Type of plant" , "Plant with featherlike leaves" , "A frond is its leaf" , "Tracheophyte plant" , "Non-flowering plant" .)