Good aside by English emperor (9)
I believe the answer is:
bonaparte
'emperor' is the definition.
(Napoleon Bonaparte)
'good aside by english' is the wordplay.
'good' becomes 'bon' (I've seen this in other clues).
'aside' becomes 'apart' (synonyms).
'by' means one lot of letters go next to another.
'english' becomes 'E' (abbreviation).
'bon'+'apart'+'e'='BONAPARTE'
(Other definitions for bonaparte that I've seen before include "Corsican family" , "Napoleon . . . . . . . . ., the little corporal" , "Exiled emperor" , "French costume drama may involve me" , "Old leader" .)