Slip up during attempt to produce a fortified wine (6)
I believe the answer is:
sherry
'a fortified wine' is the definition.
(I know that Sherry is a type of fortified wine)
'slip up during attempt to' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'during' means one lot of letters goes inside another.
'attempt to' becomes 'shy' (I've seen this in other clues).
'err' placed within 'shy' is 'SHERRY'.
'produce' acts as a link.
(Other definitions for sherry that I've seen before include "Alcoholic appetiser associated with Jerez" , "Wine from Spain's Jerez region, drunk as an aperitif" , "Fortified 1 down [WINE]" , "Alcohol for the trifle" , "Strong sweet wine as aperitif" .)