Control river pests (6)
I believe the answer is:
police
'control' is the definition.
(synonyms)
'river pests' is the wordplay.
'river' becomes 'po' (Italian river).
'pests' becomes 'lice' (lice are pests).
'po'+'lice'='POLICE'
(Other definitions for police that I've seen before include "Law-and-order force" , "Constabulary" , "Limbs of the law" , "Gardai" , "Keep watch on" .)