Free the 'orse outside (8)

I believe the answer is:

untether

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'free the 'orse outside' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.

'free the 'orse outside' is the wordplay.
'free' indicates an anagram.
''orse' becomes ''unter' ('hunter' - I've seen this before - said with a cockney accent, dropping the 'H', like 'horse' was).
'outside' means one lot of letters goes inside another.
'the' is an anagram of 'eth'.
'eth' placed inside 'unter' is 'UNTETHER'.

Can you help me to learn more?

(Other definitions for untether that I've seen before include "Release (tied animal)" , "Set free" .)

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