Very easy task collecting river junk (5)
I believe the answer is:
dross
'junk' is the definition.
'very easy task collecting river' is the wordplay.
'very easy task' becomes 'doss'.
'collecting' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'doss' placed around 'r' is 'DROSS'.
(Other definitions for dross that I've seen before include "Something of inferior quality" , "Impurity" , "Waste coal - rubbish" , "Anything worthless" , "Worthless or dangerous material" .)