Stop others following a river (6)
I believe the answer is:
arrest
'stop' is the definition.
('arrest' can be a synonym of 'stop')
'others following a river' is the wordplay.
'others' becomes 'rest' (I've seen this before).
'following' says to put letters next to each other.
'river' becomes 'r'.
'a'+'r'='ar'
'rest' after 'ar' is 'ARREST'.
(Other definitions for arrest that I've seen before include "Sartre (anag) -- check" , "Seize and take into custody" , "Stop (on suspicion)" , "Detain under suspicion of criminal behaviour" , "Catch" .)