Plant found by river in bog (4)
I believe the answer is:
fern
'plant' is the definition.
(fern is a kind of plant)
'river in bog' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'in' is an insertion indicator.
'bog' becomes 'fen' (I've seen this in another clue).
'r' put within 'fen' is 'FERN'.
'found by' acts as a link.
(Other definitions for fern that I've seen before include "Type of plant" , "Bracken, for example" , "Plant reproducing by spores" , "Girl - bracken" , "It has feathery or leafy fronds" .)