Horse shakes off one bird (5)

I believe the answer is:

heron

I'm a little stuck... Click here to teach me more about this clue!

'one bird' is the definition.
(heron is a kind of bird)

I cannot understand how the remainder of the clue works.

(Other definitions for heron that I've seen before include "Big bird" , "Tall wading bird" , "Grey or white long-legged wading bird" , "eg Egret" , "Graceful grey bird with long neck" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app