Nutmeg's supporting commercial plugging superb college (7)
I believe the answer is:
academy
'college' is the definition.
(both can mean an educational institution)
'nutmeg's supporting commercial plugging superb' is the wordplay.
'nutmeg's' becomes 'my' (where the crossword setter is 'Nutmeg').
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'commercial' becomes 'ad' (abbreviation for advertisement).
'plugging' is an insertion indicator (I've seen this in other clues).
'superb' becomes 'ace' (ace can informally mean very good).
'ad' inserted inside 'ace' is 'acade'.
'my' put after 'acade' is 'ACADEMY'.
(Other definitions for academy that I've seen before include "training centre" , "Centre of learning" , "Secondary school" , "Institution for the advancement of the arts and sciences" , "Learning institution" .)