Firmly established equine facility (6)

I believe the answer is:

stable

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'firmly established' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for stable that I've seen before include "Home for horses" , "firmly tied on?" , "Firmly balanced or fixed" , "Jesus was born in one" , "Maintaining equilibrium" .)

I've seen this clue in The Sun.

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