Tread heavily in river that's burst (7)
I believe the answer is:
explode
'burst' is the definition.
(synonyms)
'tread heavily in river' is the wordplay.
'tread heavily' becomes 'plod' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'river' becomes 'exe' (River Exe).
'plod' inserted within 'exe' is 'EXPLODE'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for explode that I've seen before include "Burst outward" , "Detonate" , "Burst violently" , "mushroom" , "Blow up" .)