River feeds bog plant (4)
I believe the answer is:
Here is my best explanation:
'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 reproducing by spores", "It has feathery or leafy fronds", "Bracken, for example", "Flowerless plant with feathery green leaves, common on mountains", "Non-flowering plant", "Type of plant", "Feathery-leaved plant", "Plant with feathery leaves", "Tracheophyte plant" and "Plant such as bracken".)