Bait worm put in river in overwhelming number (6)
I believe the answer is:
deluge
'river in overwhelming number' is the definition.
'deluge' can be an answer for 'river' (thesaurus). I am not sure about the 'in overwhelming number' bit.
'bait worm put in' is the wordplay.
'bait' becomes 'dee' (I am not sure about this - if you are sure you should believe this answer much more).
'worm' becomes 'lug' (lug is a kind of worm).
'put in' indicates putting letters inside.
'dee' placed around 'lug' is 'DELUGE'.
Can you help me to learn more?
(Other definitions for deluge that I've seen before include "Cloudburst, downpour" , "Huge downpour" , "Great flood" , "Swamp" , "Inundation" .)