Quickly dispose of the remainder in the river (6)
I believe the answer is:
presto
'quickly' is the definition.
(I've seen this before)
'remainder in the river' is the wordplay.
'remainder' becomes 'rest' (synonyms).
'in the' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'rest' put into 'po' is 'PRESTO'.
'dispose of the' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for presto that I've seen before include "at great speed..." , "Hey, it's a magician's word" , "Moving quickly" , "To be played quickly" , "(In music) fast" .)