Go off to walk in river (7)
I believe the answer is:
explode
'go off' is the definition.
(exploding is a kind of going off)
'walk in river' is the wordplay.
'walk' becomes 'plod' (plodding is a kind of walking**).
'in' is an insertion indicator.
'river' becomes 'exe' (I've seen this before).
'plod' put inside 'exe' is 'EXPLODE'.
'to' is the link.
(Other definitions for explode that I've seen before include "Detonate" , "Blow up" , "Do expel (anag)" , "Pop" , "Violently shatter" .)