Animal in stream heading west (4)
I believe the answer is:
Here is my best explanation:
'animal' is the definition.
'stream heading west' is the subsidiary indication.
'stream' becomes 'flow' ('flow' can be a synonym of 'stream').
'heading west' is a reversal indicator.
'flow' reversed gives 'wolf'.
'in' is the link.
(Other definitions for wolf that I've seen before include "Lothario", "shovel down", "Seducer - gobble down", "character of Prokofiev's music", "Predatory canine (or man?)", "22A [WILD DOG]", "Lupine animal", "Pack-hunting carnivore", "seducer" and "Austrian composer".)