Embarrassed about mistake getting possible aperitif (6)
I believe the answer is:
sherry
'possible aperitif' is the definition.
(sherry may be drunk as an aperitif)
'embarrassed about mistake' is the wordplay.
'embarrassed' becomes 'shy' (shy can mean reserved or embarrassed).
'about' is an insertion indicator.
'mistake' becomes 'err' (synonyms).
'shy' going around 'err' is 'SHERRY'.
'getting' is the link.
(Other definitions for sherry that I've seen before include "Fortified wine as aperitif" , "Alcoholic appetiser associated with Jerez" , "Wine from Spain's Jerez region, drunk as an aperitif" , "Preprandial drink" , "Fortified wine from Jerez" .)