Emperor certainly not impressing queen (4)
I believe the answer is:
nero
'emperor' is the definition.
(Roman emperor)
'certainly not impressing queen' is the wordplay.
'certainly not' becomes 'no'.
'impressing' is an insertion indicator (impress historically meant to conscript).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
'no' placed around 'er' is 'NERO'.
(Other definitions for nero that I've seen before include "Roman arsonist and fiddler" , "Cruel Roman emperor" , "Roman Emperor notorious for vice and luxury" , "Ancient Roman emperor" , "Roman emperor, d. AD68" .)