Bachelor ate a drug cocktail (8)
I believe the answer is:
graduate
'bachelor' is the definition.
(thesaurus)
'ate a drug cocktail' is the wordplay.
'cocktail' indicates an anagram.
'ate'+'a'+'drug'='ateadrug'
'ateadrug' with letters rearranged gives 'GRADUATE'.
(Other definitions for graduate that I've seen before include "Alumnus of a college" , "Holder of university degree" , "Gain degree" , "Holder of an academic degree" , "Scholar" .)