Fast horses (6)
I believe the answer is:
stable
I believe this is a double definition.
'fast' is the first definition.
(fast can mean secure)
'horses' is the second definition.
(I've seen this before)
(Other definitions for stable that I've seen before include "On an even keel" , "Firmly established" , "Horse accommodation" , "Sure!" , "Firm - building for horses" .)