Cruel person for the most part is not sweet (4)
I believe the answer is:
brut
'not sweet' is the definition.
(I've seen this in other clues)
'cruel person for the most part' is the wordplay.
'cruel person' becomes 'brute' (I've seen this before).
'for the most part' means to remove the last letter (most but not all of the letters).
'brute' with its final letter taken off is 'BRUT'.
'is' is the link.
(Other definitions for brut that I've seen before include "Dry (of champagne)" , "Of champagne, not sweet" , "(Of sparkling wine) extremely dry" , "(Of sparkling wines) very dry" , "one type of champagne" .)