Animal in stream heading west (4)
I believe the answer is:
wolf
'animal' is the definition.
(I have seen 'Predatory animal' mean 'wolf' so perhaps 'animal' could also mean 'wolf')
'stream heading west' is the wordplay.
'stream' becomes 'flow' ('flow' can be a synonym of 'stream').
'heading west' says the letters should be written in reverse.
'flow' back-to-front is 'WOLF'.
'in' acts as a link.
(Other definitions for wolf that I've seen before include "Predator that hunts in packs" , "Lothario" , "Lupine animal" , "Predatory canine (or man?)" , "Lupine mammal" .)