River going into low swamp (5)
I believe the answer is:
drown
'swamp' is the definition.
(I've seen this before)
'river going into low' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'going into' indicates putting letters inside.
'low' becomes 'down' (associated in meaning).
'r' inserted inside 'down' is 'DROWN'.
(Other definitions for drown that I've seen before include "Submerge, maybe fatally" , "Flood; kill" , "Die by immersion in liquid" , "Die in sea" , "Perish in water" .)