Takes advantage of the banks on USA river (4)
I believe the answer is:
user
'takes advantage' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'the banks on usa river' is the wordplay.
'the banks on usa' becomes 'use' (I can't explain this - if you can you should believe this answer much more).
'river' becomes 'r'.
'use'+'r'='USER'
'of' acts as a link.
(Other definitions for user that I've seen before include "One who employs" , "Patron" , "One operating something" , "Operator; drug addict" , "Junkie's" .)