Flowerless plant in bog surrounding river (4)
I believe the answer is:
fern
'flowerless plant' is the definition.
(I've seen this in another clue)
'bog surrounding river' is the wordplay.
'bog' becomes 'fen' (I've seen this in another clue).
'surrounding' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'fen' going around 'r' is 'FERN'.
'in' is the link.
(Other definitions for fern that I've seen before include "Girl - bracken" , "Bracken, for example" , "Plant such as bracken" , "Plant reproducing by spores" , "Plant with featherlike leaves" .)