Man Roman Emperor beheaded (6)
I believe the answer is:
adrian
'man' is the definition.
(I've seen this before)
'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 taken off is 'ADRIAN'.
(Other definitions for adrian that I've seen before include "The Secret Diary of ... Mole, Aged 13? (Sue Townsend)" , "Name of Roman emperor and only English pope" , "Man" , "Pope, say" , "Man's name" .)