Run off, making departure across river (6)
I believe the answer is:
decamp
'run off' is the definition.
('decamp' can be a synonym of 'run off')
'departure across river' is the wordplay.
'departure' becomes 'dep'.
'across' means one lot of letters goes inside another.
'river' becomes 'cam' (I've seen this before).
'dep' placed around 'cam' is 'DECAMP'.
'making' is the link.
(Other definitions for decamp that I've seen before include "Make off - from tent?" , "escape" , "Split" , "Make off secretly" , "Fold up tents and depart" .)