Go off to walk in river (7)
I believe the answer is:
Here is my best explanation:
'go off' is the definition.
(exploding is a kind of going off)
'walk in river' is the subsidiary indication.
'walk' becomes 'plod' (plodding is a kind of walking**).
'in' is an insertion indicator.
'river' becomes 'exe'.
'plod' placed within 'exe' is 'explode'.
'to' is the link.
(Other definitions for explode that I've seen before include "Burst violently", "Violently shatter", "Detonate", "Blow up", "Do expel (anag)", "Burst outward", "Go off with a bang" and "Pop".)