Ruin cellar's top brandy (4)
I believe the answer is:
marc
'brandy' is the definition.
(I know that marc is a type of brandy)
'ruin cellar's top' is the wordplay.
'ruin' becomes 'mar' (I've seen this before).
'top' indicates taking the first letters (in a down clue, the first letter is top).
The first letter of 'cellars' is 'c'.
'mar'+'c'='MARC'
(Other definitions for marc that I've seen before include "Brandy made from wine press residue" , "Spirit" , "Drink" , "Grape refuse" , "Sort of brandy made from the remains of pressed grapes" .)