The most common and most likely answer to this crossword clue is the 6 letter word BADASS
I've seen this crossword clue in The Guardian
B
A
D
A
S
S
Buddy explains!
The straight part
We know that "American tough guy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case american tough guy = badass.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 BADASS mean?
BADASS noun- (negative connotation) A belligerent or mean person; a person with an unpleasantly extreme appearance, attitudes, or behavior.
- (youth slang, positive connotation) A person considered impressive due to courage, skill, and/or toughness.
- (negative connotation) Belligerent and troublesome.
- (positive connotation) Having an extreme appearance, attitude, or behavior that is considered admirable.