Criminals in group ultimately pollute this river (6)
I believe the answer is:
ganges
'criminals in group ultimately pollute this river' is the definition.
'Ganges' can be an answer for 'river' (Ganges is an example). I am not certain of the remainder of the definition.
'group ultimately pollute this river' is the wordplay.
'group' becomes 'gang' (a gang is a group of people).
'ultimately' suggests the final letters.
'this river' becomes 's' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'pollute' is 'e'.
'gang'+'e'+'s'='GANGES'
Can you help me to learn more?
(Other definitions for ganges that I've seen before include "Holy river on India" , "Indian flower" , "Sacred Hindu river in India" , "Sacred river of N India" , "Great river of India" .)