Tank churned up erosive rift between rivers (9)
I believe the answer is:
reservoir
'tank' is the definition.
(I know that reservoir is a type of tank)
'churned up erosive rift between rivers' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'erosive' is 'eservoi' which is located in the answer.
'rivers' could be 'rr' (an abbreviation for river written twice) and 'rr' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for reservoir that I've seen before include "water over a wide area?" , "Area flooded" , "Man-made lake of stored water" , "Lake created to store water" , "Lake for collecting and storing water" .)