River feeds bog plant (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'river feeds bog' is the wordplay.
'river' becomes 'r'.
'feeds' means one lot of letters goes inside another.
'bog' becomes 'fen'.
'r' placed inside 'fen' is 'FERN'.
(Other definitions for fern that I've seen before include "Plant such as bracken" , "Plant with featherlike leaves" , "Non-flowering plant" , "Plant producing spores" , "Tracheophyte plant" .)