Go on walk by river (6)
I believe the answer is:
ramble
'go on' is the definition.
(I've seen this before)
'walk by river' is the wordplay.
'walk' becomes 'amble' (amble is a kind of walk**).
'by' says to put letters next to each other.
'river' becomes 'r'.
'amble' put after 'r' is 'RAMBLE'.
(Other definitions for ramble that I've seen before include "Walk in the country - talk aimlessly at length" , "Wander where you please" , "Show incoherence" , "Walk as leisure activity" , "Walk; be discursive" .)