Animal jumping two ducks on river (3)
I believe the answer is:
roo
'animal' is the definition.
(I've seen this before)
'jumping two ducks on river' is the wordplay.
'jumping' says the letters should be written in reverse (in a down clue, letters jump up).
'two ducks' becomes 'oo'.
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'oo'+'r'='oor'
'oor' written backwards gives 'ROO'.
(Other definitions for roo that I've seen before include "Kanga's child (Pooh)" , "Marsupial (Aus. colloq.)" , "large animal" , "Hopper" , "bounder" .)