Wanting untouchable's brevity (9)

I believe the answer is:

shortness

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

'wanting untouchable's' is the wordplay.
'wanting' becomes 'short' (I've seen this before).
'untouchable's' becomes 'ness' (I can't justify this - if you can you should believe this answer much more).
'short'+'ness'='SHORTNESS'

Can you help me to learn more?

(Another definition for shortness that I've seen is " Brevity".)

I've seen this clue in the Sydney Morning Herald.

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