Ratty, I rub out river that flows into another (9)
I believe the answer is:
tributary
'river that flows into another' is the definition.
'tributary' can be an answer for 'river' (I've seen this before). I am unsure of the remainder of the definition.
'ratty i rub out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'ratty'+'i'+'rub'='rattyirub'
'rattyirub' is an anagram of 'TRIBUTARY'.
Can you help me to learn more?
(Other definitions for tributary that I've seen before include "water feeder" , "River feeding a larger one" , "revenue from clients" , "River that feeds a larger one" , "River flowing into a larger one" .)