Pull into river swamp (6)
I believe the answer is:
deluge
'swamp' is the definition.
(synonyms)
'pull into river' is the wordplay.
'pull' becomes 'lug' (I've seen this before).
'into' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'lug' put into 'dee' is 'DELUGE'.
(Other definitions for deluge that I've seen before include "Overwhelm with water" , "Flooding downpour" , "Flood - downpour" , "Inundation" , "A heavy downpour of rain" .)