Drunk with added power — it goes to one's head (6)
I believe the answer is:
topper
'it goes to one's head' is the definition.
(topper can mean a top hat)
'drunk with added power' is the wordplay.
'drunk' becomes 'toper' (toper can mean a drunken person).
'with added' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'toper' going around 'p' is 'TOPPER'.
(Other definitions for topper that I've seen before include "Formal headgear" , "Informally a tall hat - the best" , "Top hat (colloq.)" , "Hat for formal occasions" , "lid" .)