The most common and most likely answer to this crossword clue is the 4 letter word SOUR
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More clues leading to the result SOUR
Buddy explains!

The straight part
We know that "Cocktail" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cocktail = sour.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SOUR mean?
SOUR noun- The sensation of a sour taste.
- A drink made with whiskey, lemon or lime juice and sugar.
- (by extension) Any cocktail containing lemon or lime juice.
- A sour or acid substance; whatever produces a painful effect.
- The acidic solution used in souring fabric.
- To make sour.
- To become sour.
- To spoil or mar; to make disenchanted.
- To become disenchanted.
- To make (soil) cold and unproductive.
- To macerate (lime) and render it fit for plaster or mortar.
- To process (fabric) after bleaching, using hydrochloric acid or sulphuric acid to wash out the lime.
- Having an acidic, sharp or tangy taste.
- Made rancid by fermentation, etc.
- Tasting or smelling rancid.
- (of a person's character) Peevish or bad-tempered.
- (of soil) Excessively acidic and thus infertile.
- (of petroleum) Containing excess sulfur.
- Unfortunate or unfavorable.
- Off-pitch, out of tune.