Students sauntered around having swallowed a drug naughtily (14)

'students' is the definition.
(undergraduate is a kind of student)

'sauntered around having swallowed a drug naughtily' is the wordplay.
'around' indicates anagramming the letters.
'having swallowed' indicates putting letters inside.
'naughtily' is an anagram indicator.
'sauntered' anagrammed gives 'underates'.
'a'+'drug'='adrug'
'adrug' with letters rearranged gives 'gradu'.
'underates' enclosing 'gradu' is 'UNDERGRADUATES'.