Prove you are the boss - as Henry VIII did (6)
I believe the answer is:
behead
'henry viii did' is the definition.
I can't judge whether this definition defines the answer.
'prove you are the boss' is the wordplay.
'prove you' becomes 'be' (I am not sure about this - if you are sure you should believe this answer much more).
'are the boss' becomes 'head' (I've seen this before).
'be'+'head'='BEHEAD'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for behead that I've seen before include "Knock one's block off!" , "Guillotine" , "Knock someone's block off?" , "Remove the crown" , "wield the axe" .)