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

The straight part
We know that "A stopper" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a stopper = bung.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 BUNG mean?
BUNG noun- A stopper, alternative to a cork, often made of rubber used to prevent fluid passing through the neck of a bottle, vat, a hole in a vessel etc.
- A cecum or anus, especially of a slaughter animal.
- A bribe.
- The orifice in the bilge of a cask through which it is filled; bunghole.
- A sharper or pickpocket.
- To plug, as with a bung.
- To put or throw somewhere without care; to chuck.
- To batter, bruise; to cause to bulge or swell.
- To pass a bribe.
- Broken, not in working order.
- (thieves' cant) A purse.