Ruin drink at border (6)
I believe the answer is:
margin
'border' is the definition.
(I know that border can be written as margin)
'ruin drink' is the wordplay.
'ruin' becomes 'mar' (I've seen this before).
'drink' becomes 'gin' (gin is an alcoholic drink).
'mar'+'gin'='MARGIN'
'at' acts as a link.
(Other definitions for margin that I've seen before include "Boundary line or the area" , "Permissible difference" , "Blank space around a page" , "Periphery" , "Border or leeway" .)