Hit barrier on river to the north (6)
I believe the answer is:
wallop
'hit' is the definition.
(walloping is a kind of hitting)
'barrier on river to the north' is the wordplay.
'barrier' becomes 'wall' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'river' becomes 'po' (Po is an example).
'to the north' shows that the letters should be reversed in order.
'po' back-to-front is 'op'.
'wall'+'op'='WALLOP'
(Other definitions for wallop that I've seen before include "Best" , "big hit" , "Alcoholic drink" , "Punch" , "A heavy resounding blow (6)" .)