Reliable outbuilding (6)
I believe the answer is:
stable
This clue is a double definition.
'reliable' is the first definition.
(thesaurus)
'outbuilding' is the second definition.
(Other definitions for stable that I've seen before include "Firm - building for horses" , "Firmly established" , "Housing for a horse" , "Steady - home for dobbin" , "Maintaining equilibrium" .)