Stage one in reversing seasonal disorder (4)
I believe the answer is:
dais
'stage' is the definition.
(type of platform or stage)
'one in reversing seasonal disorder' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' indicates putting letters inside.
'reversing' is a reversal indicator.
'seasonal disorder' becomes 'sad' (I can't justify this - if you can you should give a lot more credence to this answer).
'sad' reversed gives 'das'.
'i' placed within 'das' is 'DAIS'.
Can you help me to learn more?
(Other definitions for dais that I've seen before include "Throne platform" , "stand" , "stage" , "Piece of old cloth" , "device to give some an uplift at conference?" .)