Kick the horse out, lacking steadiness (8)
I believe the answer is:
unstable
If this is the answer, I'm afraid I cannot explain it.
Can you help me to learn more?
(Other definitions for unstable that I've seen before include "Prone to change" , "Moody" , "Not well-balanced, liable to wobble" , "Precarious or subject to change" , "not reliable" .)