The most common and most likely answer to this crossword clue is the 4 plus 3 letter word JOHN DOE
I've seen this crossword clue in The Times
J
O
H
N
D
O
E
More clues leading to the result JOHN DOE
Buddy explains!
The straight part
We know that "Unknown American fellow" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unknown american fellow = john doe.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.