Derby cocktail (5)
I believe the answer is:
julep
'derby cocktail' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for julep that I've seen before include "Cocktail of bourbon, sugar and mint over crushed ice" , "Sweet drink, often medicated" , "(Minted) drink made with bourbon, sugar and crushed ice" , "See 21 Down" , "Mint drink in America" .)