Monster wants patriarch and prophet beheaded (9)
I believe the answer is:
leviathan
'monster' is the definition.
(leviathan is a kind of monster)
'patriarch and prophet beheaded' is the wordplay.
I cannot really 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 present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
This explanation may well be incorrect...
'wants' is the link.
Can you help me to learn more?
(Other definitions for leviathan that I've seen before include "Biblical sea monster" , "Gigantic" , "Monstrous sea creature" , "An evil hat turns to anything of great size" , "Huge (sea) monster" .)