Mature former pupil promoting wine shop (6)
I believe the answer is:
bodega
'wine shop' is the definition.
(I've seen this before)
'mature former pupil promoting' is the wordplay.
'mature' becomes 'aged' (similar in meaning).
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'promoting' is a reversal indicator (I've seen 'promoted' mean this (going up, in a down clue)).
'aged'+'ob'='agedob'
'agedob' written backwards gives 'BODEGA'.
(Other definitions for bodega that I've seen before include "Small Spanish grocery and wine store" , "Small shop in Spain selling wine and groceries" , "Small Hispanic wine-shop" , "Liquor store" , "Wine-shop in a Spanish-speaking country" .)