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

The straight part
We know that "thug" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thug = goon.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 GOON mean?
GOON noun- A thug; a usually muscular henchman with little intelligence (also known as a 'hired goon').
- A fool; someone considered silly, stupid, awkward, or outlandish.
- An enforcer or fighter.
- (WWII, PoW slang) A German guard in a prisoner-of-war camp.
- A member of the comedy web site Something Awful.
- A wine flagon or cask.
- Cheap or inferior cask wine.