Sit so upright during physical education (4)
I believe the answer is:
pose
'sit' is the definition.
(as in sitting for a painter)
'so upright during physical education' is the wordplay.
'upright' shows that the letters should be reversed in order (in a down clue, letters go up).
'during' means one lot of letters goes inside another.
'physical education' becomes 'pe' ().
'so' in reverse letter order is 'os'.
'os' going within 'pe' is 'POSE'.
(Other definitions for pose that I've seen before include "Arrange, model" , "Model; attitude" , "Strike an attitude as for photo" , "Pretend; model" , "Sit; attitude" .)