Emperor's granny concealing Eastern European origins, originally (8)
I believe the answer is:
Here is my best explanation:
'emperor's' is the definition.
(Napoleon is an example)
'granny concealing eastern european origins originally' is the subsidiary indication.
'granny' becomes 'nan'.
'concealing' indicates putting letters inside.
'eastern european' becomes 'pole'.
'origins' says to take the initial letters.
The initial letter of 'originally' is 'o'.
'nan' placed around 'poleo' is 'napoleon'.
(Other definitions for napoleon that I've seen before include "Frenchman known as The Little Corporal", "Head pig on Animal Farm", "Gold twenty franc piece", "No one, pal, matches this Little Corporal", "Old French coin", "Cake", "Corsican conqueror", "leader in France, once", "Leading pig in Animal Farm" and "no longer tender".)