So excited one couldn't keep still? (7)

I believe the answer is:

stirred

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'so excited' is the definition.
'stirred' can be an answer for 'excited' (similar in meaning). I am not certain of the 'so' bit.

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for stirred that I've seen before include "Rose from sleep" , "Excited" , "Aroused; agitated" , "Emotionally aroused" , "Moved slightly" .)

I've seen this clue in the Evening Standard.

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