Awful leaders of dictatorships, in Roman emperors (4)
I believe the answer is:
dire
'awful' is the definition.
(I know that awful can be written as dire)
'leaders of dictatorships in roman emperors' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'dictatorships in roman emperors' is 'DIRE'.
(Other definitions for dire that I've seen before include "'Dreadful, terrible (4)'" , "Ominous; very bad" , "Nearly hopeless" , "Very serious indeed" , "Fearful" .)