Slip up during attempt to make fortified wine (6)
I believe the answer is:
sherry
'fortified wine' is the definition.
(sherry is a kind of fortified wine)
'slip up during attempt' is the wordplay.
'slip up' becomes 'err' (to err is to make an error).
'during' is an insertion indicator.
'attempt' becomes 'shy' (I've seen this in other clues).
'err' put into 'shy' is 'SHERRY'.
'to make' acts as a link.
(Other definitions for sherry that I've seen before include "Spanish fortified wine" , "Spanish drink" , "Fortified wine as aperitif" , "Amontillado, e.g." , "booze" .)