Treated with sulphur, alien dies horribly (10)

I believe the answer is:

vulcanised

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'treated with sulphur alien dies horribly' is the definition.
I can't tell whether this defines the answer.

'with sulphur alien dies horribly' is the wordplay.
'with' indicates putting letters inside.
'sulphur alien' becomes 'vulcan' (I can't justify this - if you can you should believe this answer much more).
'horribly' is an anagram indicator (a horribly wrong version of the letters).
'dies' with letters rearranged gives 'ised'.
'vulcan' put within 'ised' is 'VULCANISED'.

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I've seen this clue in The Independent.

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