Undistinguished former pupil certain to ingest cocaine (7)
I believe the answer is:
obscure
'undistinguished' is the definition.
(thesaurus)
'former pupil certain to ingest cocaine' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'certain' becomes 'sure' (synonyms).
'to ingest' indicates putting letters inside (inserted letters are ingested or eaten).
'cocaine' becomes 'C' (drugs abbreviation).
'sure' placed around 'c' is 'scure'.
'ob'+'scure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Jude may be" , "Overshadow or eclipse" , "Vague - mask" , "Indistinct or not well-known" , "Not clear, hidden" .)