Occult Mass in unorthodox surroundings (10)

I believe the answer is:

hermetical

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'surroundings' is the definition.
The definition and answer are not the same part of speech.

'occult mass in unorthodox' is the wordplay.
'occult mass' becomes 'm' (symbol used in Physics e.g. f=ma. I am not sure about the 'occult' bit.).
'in' is an insertion indicator.
'unorthodox' becomes 'heretical' (similar in meaning).
'm' put into 'heretical' is 'HERMETICAL'.

Can you help me to learn more?

(Another definition for hermetical that I've seen is " sealed".)

I've seen this clue in The Times.

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