K? (8)
'k?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
This is all the clue.
(Other definitions for cocktail that I've seen before include "Screwdriver, for example" , "Possibly old-fashioned" , "old-fashioned in America" , "snowball?" , "Manhattan, for example" .)
(Other definitions for rock bottom that I've seen before include "Lowest possible level" , "As low as you can get" , "Hard bed" , "worst condition possible" , "such a price is heavily reduced" .)