Refuse by river (5)
I believe the answer is:
offal
'refuse' is the definition.
(I've seen this in other clues)
'by river' is the wordplay.
'by' becomes 'of' (I am not sure about this - if you are sure you should believe this answer much more).
'river' becomes 'fal' (River Fal).
'of'+'fal'='OFFAL'
Can you help me to learn more?
(Other definitions for offal that I've seen before include "Refuse - edible entrails of animal" , "Edible (or inedible!) animal organs" , "Internal organs of an animal used as food" , "Animal entrails and internal organs - worthless" , "Putting cargo on (ship)" .)