Split between top of the bank and length of the river (6)
I believe the answer is:
breach
'split' is the definition.
(I've seen this before)
'top of the bank and length of the river' is the wordplay.
I cannot quite see how this works, but
'length' could be 'r' (r is a kind of length) and 'r' is found within the answer.
'river' could be 'ea' (I've seen this in other clues) and 'ea' is found in the answer.
This explanation may well be incorrect...
'between' acts as a link.
Can you help me to learn more?
(Other definitions for breach that I've seen before include "Gap (in a wall/barrier)" , "Severance" , "Split, break" , "Gap made in a fortification" , "Opening -- infraction" .)