Whisky on a Mediterranean island (5)
I believe the answer is:
malta
'mediterranean island' is the definition.
(Malta is an island in the Mediterranean)
'whisky on a' is the wordplay.
'whisky' becomes 'malt' (malt whisky is a kind of whisky).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'malt'+'a'='MALTA'
(Other definitions for malta that I've seen before include "Mediterranean country" , "defiant island in WW2" , "Mediterranean island, capital Valletta" , "Mediterranean island state" , "small island" .)