Most of river reaches English channel eventually (2,3,6)
I believe the answer is:
in due course
'eventually' is the definition.
(I've seen this in another clue)
'most of river reaches english channel' is the wordplay.
'most of river reaches' becomes 'indu' (I can't justify this - if you can you should give a lot more credence to this answer).
'english' becomes 'e' (abbreviation).
'channel' becomes 'course' (I've seen this before).
'indu'+'e'+'course'='IN DUE COURSE'
(Other definitions for in due course that I've seen before include "When the time comes" , "at the proper time" , "Eventually" , "At the appropriate time" , "in the natural order" .)