Ruler in crown drinking last drop of Barolo wine (9)
I believe the answer is:
potentate
'ruler' is the definition.
(I know that potentate is a type of ruler)
'crown drinking last drop of barolo wine' is the wordplay.
'crown' becomes 'pate' (both can mean a person's head).
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'last drop of' suggests the final letters.
'wine' becomes 'tent' (type of Spanish wine).
The last letter of 'barolo' is 'o'.
'o'+'tent'='otent'
'pate' going around 'otent' is 'POTENTATE'.
'in' acts as a link.
(Other definitions for potentate that I've seen before include "Powerful autocratic ruler" , "A monarch or ruler" , "Mogul" , "authority" .)