Unhappy about a standard deviation (3)

I believe the answer is:

sad

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'unhappy' is the definition.
(synonyms)

'about a standard deviation' is the wordplay.
'about' indicates putting letters inside.
'standard deviation' becomes 'sd' (this could be a standard abbreviation which I don't know about).
'a' placed within 'sd' is 'SAD'.

Can you help me to learn more?

(Other definitions for sad that I've seen before include "Melancholy, downhearted" , "Miserable" , "upset" , "Doleful" , "Melancholy, depressed" .)

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

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