Once summoned to appear at the court, is keyed-up (7)

I believe the answer is:

excited

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

'once summoned' is the wordplay.
'once' becomes 'ex' (prefix meaning former or onetime).
'summoned' becomes 'cited' ('cite' can be a synonym of 'summon').
'ex'+'cited'='EXCITED'

'to appear at the court is keyed' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for excited that I've seen before include "Stimulated" , "Worked up" , "Roused by strong emotion" , "Hot" , "Aroused; eager" .)

I've seen this clue in the Evening Standard.

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