All others, younger, fidgety (8)

I believe the answer is:

restless

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'fidgety' is the definition.
('restless' can be a synonym of 'fidgety')

'all others younger' is the wordplay.
'all others' becomes 'rest' (as in the rest of a group).
'younger' becomes 'less' (I can't explain this - if you can you should believe this answer much more).
'rest'+'less'='RESTLESS'

Can you help me to learn more?

(Other definitions for restless that I've seen before include "Unable to sit still" , "Unable to relax or be still" , "Unable to keep still" , "Uneasy and agitated" , "Fidgety - unable to sleep" .)

I've seen this clue in The Telegraph.

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