Trace river dividing Slough (5)
I believe the answer is:
shred
'trace' is the definition.
(I've seen this in another clue)
'river dividing slough' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'dividing' indicates putting letters inside (inserted letters divide the word).
'slough' becomes 'shed' (both can mean to discard something).
'r' put inside 'shed' is 'SHRED'.
(Other definitions for shred that I've seen before include "(Tear into) small piece(s)" , "Scrap; finely tear" , "Cut into tiny pieces" , "Small piece torn from something" , "Strip of material; a very small amount" .)