Control river parasites (6)
I believe the answer is:
police
'control' is the definition.
(I've seen this before)
'river parasites' is the wordplay.
'river' becomes 'po' (Italian river).
'parasites' becomes 'lice' (I've seen this before).
'po'+'lice'='POLICE'
(Other definitions for police that I've seen before include "Constabulary" , "Law enforcers" , "Emergency service" , "Limbs of the law" , "Gardai" .)