Ruler on plane sick having eaten duck (8)
I believe the answer is:
napoleon
'ruler' is the definition.
(Napoleon is an example)
'on plane sick having eaten duck' is the wordplay.
'sick' indicates anagramming the letters.
'having eaten' indicates putting letters inside.
'duck' becomes 'o'.
'on'+'plane'='onplane'
'onplane' anagrammed gives 'napolen'.
'napolen' going around 'o' is 'NAPOLEON'.
(Other definitions for napoleon that I've seen before include "no longer tender" , "Cake" , "French emperor, d.1821" , "Head pig on Animal Farm" , "Open loan to the French general. strangely" .)