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 "Frenchman known as The Little Corporal", "Old French coin", "Corporal and emperor", "French emperor, d.1821", "Leading pig in Animal Farm", "The familiar Bonaparte", "Former French gold coin", "French Emperor", "Gold twenty franc piece" and "Head pig on Animal Farm".)