Ruler on plane sick having eaten duck (8)
I believe the answer is:
Here is my best explanation:
'ruler' is the definition.
(Napoleon is an example)
'on plane sick having eaten duck' is the subsidiary indication.
'sick' is an anagram indicator.
'having eaten' means one lot of letters goes inside another.
'duck' becomes 'o'.
'onplane' anagrammed gives 'napolen'.
'napolen' enclosing 'o' is 'napoleon'.
(Other definitions for napoleon that I've seen before include "French general and emperor", "Gold twenty franc piece", "The little French general", "In France, former sovereign", "Leading pig in Animal Farm", "French VIP", "Frenchman known as The Little Corporal", "Head pig on Animal Farm", "Cake" and "A pig farmer was chased by".)