Force parasites behind river (6)
I believe the answer is:
police
'force' is the definition.
(I know that police is a type of force)
'parasites behind river' is the wordplay.
'parasites' becomes 'lice' (lice are parasitic insects).
'behind' means one lot of letters go next to another.
'river' becomes 'Po' (Italian river).
'lice' put after 'po' is 'POLICE'.
(Other definitions for police that I've seen before include "Constabulary" , "Civil force" , "Long arm of the law" , "Lawmen" , "Heat in US" .)