Small charge covers river ramble (6)
I believe the answer is:
stroll
'ramble' is the definition.
(synonyms)
'small charge covers river' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'charge' becomes 'toll' (toll is a kind of charge).
'covers' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'toll' placed around 'r' is 'troll'.
's'+'troll'='STROLL'
(Other definitions for stroll that I've seen before include "swan" , "Amble along" , "Saunter in no particular direction" , "A casual or leisurely walk" , "Walk slowly and idly" .)