Adult, saucy, admitting one provided drink (8)
I believe the answer is:
aperitif
'drink' is the definition.
(aperitif is a kind of drink)
'adult saucy admitting one provided' is the wordplay.
'adult' becomes 'a' (abbreviation).
'saucy' becomes 'pert' (synonyms).
'admitting' is an insertion indicator (admit can mean to welcome in or accept).
'one' becomes 'I' (Roman numeral).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'pert' enclosing 'i' is 'perit'.
'a'+'perit'+'if'='APERITIF'
(Other definitions for aperitif that I've seen before include "it leads to blow-out" , "Alcoholic drink to stimulate appetite" , "Pre-dinner drink" , "What's drunk before a meal?" , "Alcoholic drink taken before meal" .)