Stone alien for particular skill or technique that is intellectually demanding (13)

I believe the answer is:

rocketscience

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'skill or technique that is intellectually demanding' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'stone alien for particular' is the wordplay.
'stone' becomes 'rock' (synonyms).
'alien' becomes 'et' (extra terrestrial).
'for particular' becomes 'science' (I am not sure about this - if you are sure you should believe this answer much more).
'rock'+'et'+'science'='ROCKETSCIENCE'

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I've seen this clue in The Irish Times.

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