Endless whisky on the rocks reveals bitterness (6)
I believe the answer is:
malice
'bitterness' is the definition.
(thesaurus)
'endless whisky on the rocks' is the wordplay.
'endless' means to remove the last letter.
'whisky' becomes 'malt' (malt whisky is a kind of whisky).
'on' is a charade indicator (letters next to each other).
'the rocks' becomes 'ice' (I've seen this before).
'malt' with its last letter removed is 'mal'.
'mal'+'ice'='MALICE'
'reveals' is the link.
(Other definitions for malice that I've seen before include "Spite - desire to hurt" , "Spiteful, animosity, sometimes a forethought" , "'Ill-will, spite (6)'" , "evil intent" , "Wrongful intention" .)