Firmly fixed or established (6)
I believe the answer is:
stable
I believe this clue is a double definition.
'firmly fixed' is the first definition.
(I've seen this before)
'established' is the second definition.
(thesaurus)
(Other definitions for stable that I've seen before include "Firm - building for horses" , "farm building" , "Firm and dependable" , "Not likely to change" , "Firmly fixed - home for horse" .)