Smacking His Majesty on bridge (8)
I believe the answer is:
spanking
'smacking' is the definition.
(thesaurus)
'his majesty on bridge' is the wordplay.
'his majesty' becomes 'king' (I am not sure about this - if you are sure you should believe this answer much more).
'on' says to put letters next to each other.
'bridge' becomes 'span' (synonyms).
'king' put after 'span' is 'SPANKING'.
Can you help me to learn more?
(Other definitions for spanking that I've seen before include "excellent" , "Quick and energetic (1)" , "Hiding" , "making one smart" , "Very large" .)