A tax on His Majesty’s neckwear (6)
I believe the answer is:
cravat
'neckwear' is the definition.
(cravat is a kind of neckwear)
'a tax on his majesty's' is the wordplay.
'tax' becomes 'VAT' (Value-Added Tax).
'on' means one lot of letters go next to another.
'his majesty' becomes 'CR' (Charles Rex - ie King Charles).
'a'+'vat'='avat'
'avat' after 'cr' is 'CRAVAT'.
(Other definitions for cravat that I've seen before include "A man's fancy necktie" , "A fancy old necktie" , "Accessory" , "Neck scarf" , "Menswear" .)