Stupid of His Majesty to go on donkey (5)
I believe the answer is:
crass
'stupid' is the definition.
(I've seen this before)
'his majesty to go on donkey' is the wordplay.
'his majesty' becomes 'CR' (Charles Rex - ie King Charles).
'to go on' says to put letters next to each other (in a down clue, some letters go on top of others).
'donkey' becomes 'ass' (ass is a term for a donkey).
'cr'+'ass'='CRASS'
'of' acts as a link.
(Other definitions for crass that I've seen before include "Boorish about scars" , "Tactless, boorish" , "Insensitively stupid" , "Grossly stupid, tactless" , "Totally lacking in sensibility" .)