The supreme power to make jokes during May (7)
I believe the answer is:
majesty
'the supreme power' is the definition.
(I've seen this before)
'jokes during may' is the wordplay.
'jokes' becomes 'jest' (I am not sure about this - if you are sure you should believe this answer much more).
'during' means one lot of letters goes inside another.
'jest' placed within 'may' is 'MAJESTY'.
'to make' is the link.
Can you help me to learn more?
(Other definitions for majesty that I've seen before include "Splendour" , "Royal quality" , "Stateliness" , "Supreme power" , "Monarch" .)