A Roman emperor, in a manner of speaking (4)
I believe the answer is:
nero
'a roman emperor' is the definition.
(Roman emperor, 37-68 AD)
'in a manner of speaking' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NERO' is hidden in the letters of 'manner of speaking'.
(Other definitions for nero that I've seen before include "Ruler" , "Emperor (who fiddled while Rome burned?) d. AD68" , "Roman Emperor, 54-68 AD" , "Roman Emperor notorious for vice and luxury" , "He's reported to have fiddled while Rome burned" .)