Crazy about energy drink (4)
I believe the answer is:
'drink' is the definition.
(mead is a kind of drink)
'crazy about energy' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'about' means one lot of letters goes inside another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'mad' enclosing 'e' is 'MEAD'.
(Other definitions for mead that I've seen before include "Cooked joint" , "Cornish drink" , "Drink made of fermented honey and water" , "Alcoholic drink of fermented honey and water of old" , "something to drink" .)