Pope Roman Emperor beheaded (6)
I believe the answer is:
adrian
'pope' is the definition.
(Pope Adrian)
'roman emperor beheaded' is the wordplay.
'roman emperor' becomes 'hadrian' (I've seen this before).
'beheaded' means to remove the first letter (the head of the word is removed).
'hadrian' with its initial letter removed is 'ADRIAN'.
(Other definitions for adrian that I've seen before include "The first and only English Pope" , "Sir -- Boult, conductor" , "-- Mole, Sue Townsend's creation" , "Man's name" , "Name of six Popes" .)