Sit so upright during physical education (4)

I believe the answer is:

pose

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'sit' is the definition.
(synonyms)

'upright during physical education' is the wordplay.
'upright' becomes 'os' (I can't explain this - if you can you should believe this answer much more).
'during' is an insertion indicator.
'physical education' becomes 'pe'.
'os' put within 'pe' is 'POSE'.

'so' is the link.

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(Other definitions for pose that I've seen before include "Arrange, model" , "Model; attitude" , "Strike an attitude as for photo" , "Pretend; model" , "Sit; attitude" .)

I've seen this clue in the Evening Standard.

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