Firm for horses (6)
I believe the answer is:
stable
I believe this is a double definition.
'firm' is the first definition.
(similar in meaning)
'for horses' is the second definition.
(I've seen this before)
(Other definitions for stable that I've seen before include "Lasting" , "Steady, unchanging" , "Sound" , "Durable; horse shed" , "Bleats (anag)" .)