Retiring, slip inside for a drink (6)
I believe the answer is:
sherry
'for a drink' is the definition.
(sherry is a kind of drink)
'retiring slip inside' is the wordplay.
'retiring' becomes 'shy' (both can mean reserved or introverted).
'slip' becomes 'err' (to err is to make an error).
'inside' means one lot of letters goes inside another.
'shy' enclosing 'err' is 'SHERRY'.
(Other definitions for sherry that I've seen before include "Fortified wine from Jerez" , "Wine from Spain's Jerez region, drunk as an aperitif" , "Spanish drink" , "Christmas drink?" , "Alcoholic drink" .)