Sit so upright during physical education (4)
I believe the answer is:
pose
'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.
Can you help me to learn more?
(Other definitions for pose that I've seen before include "Arrange, model" , "Model; attitude" , "Strike an attitude as for photo" , "Pretend; model" , "Sit; attitude" .)