Wants Your Majesty to interrupt Desmond casually (7)
I believe the answer is:
desires
'wants' is the definition.
('desire' can be a synonym of 'want')
'your majesty to interrupt desmond casually' is the wordplay.
'your majesty' becomes 'sire' (I've seen this in another clue).
'to interrupt' means one lot of letters goes inside another.
'desmond casually' becomes 'des' (I am not sure about this - if you are sure you should believe this answer much more).
'sire' placed within 'des' is 'DESIRES'.
Can you help me to learn more?
(Other definitions for desires that I've seen before include "needs" , "Wants (7)." , "Covets and yearns for" , "Hopes" , "Wishes" .)