Monster patriarch beheaded prophet (9)
I believe the answer is:
leviathan
'monster' is the definition.
(I know that leviathan is a type of monster)
'patriarch beheaded prophet' is the wordplay.
I cannot quite see how this works, but
'patriarch' could be 'levi' (I have seen 'Hebrew patriarch' mean 'levi' so perhaps 'patriarch' could also mean 'levi') and 'levi' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for leviathan that I've seen before include "Biblical sea-monster" , "Huge (sea) monster" , "Autocratic monarch" , "An evil hat turns to anything of great size" , "Gigantic" .)