Killing Phil, Mario and Gus? Off with their heads! (9)
I believe the answer is:
hilarious
'killing' is the definition.
(killing can informally mean very funny)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for hilarious that I've seen before include "Uriah's oil (anag.)" , "Louis' hair! (anag)" , "Extremely amusing" , "Priceless!" , "Extremely funny" .)