A polo horse gets out of the way of this plant! (5)
I believe the answer is:
peony
'plant' is the definition.
(peony is a kind of plant)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for peony that I've seen before include "Garden flower" , "Plant with large red, white or pink flowers" , "Herbaceous flower" , "Bloom" .)