Wine that in test is tastiest (4)
I believe the answer is:
asti
'wine' is the definition.
(I've seen this before)
'in test is tastiest' is the wordplay.
'in' says the answer is hidden in the clue.
'ASTI' is hidden in the letters of 'test tastiest'.
'that' acts as a link.
(Other definitions for asti that I've seen before include "Italian drunk" , "Source of fizz" , "Bubbly wine" , "Piedmontese white wine" , "Fizzy wine" .)