Dog in river leaving scent (4)
I believe the answer is:
tail
'dog' is the definition.
('tail' can be a synonym of 'dog')
'river leaving scent' is the wordplay.
'river' becomes 'r'.
'leaving' indicates named letters should be taken away.
'scent' becomes 'trail' (I've seen this before).
'trail' with 'r' removed is 'TAIL'.
'in' acts as a link.
(Other definitions for tail that I've seen before include "Dog - end" , "Follow the caudal appendage" , "Follow closely - taper (off)" , "Rear appendage of animal" , "Follow - train" .)