Simple job collecting river waste (5)
I believe the answer is:
dross
'waste' is the definition.
(I know that dross is a type of waste)
'simple job collecting river' is the wordplay.
'simple job' becomes 'doss' (I can't explain this - if you can you should give a lot more credence to this answer).
'collecting' is an insertion indicator.
'river' becomes 'r' (abbreviation for river).
'doss' going around 'r' is 'DROSS'.
Can you help me to learn more?
(Other definitions for dross that I've seen before include "Waste matter, refuse" , "Rubbish; impurities" , "Worthless matter" , "Anything worthless" , "Impurity" .)