The most common and most likely answer to this crossword clue is the 5 letter word LITER
I've seen this crossword clue in Financial Times
L
I
T
E
R
More clues leading to the result LITER
Buddy explains!

The straight part
We know that "American unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case american unit = liter.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 LITER mean?
LITER noun- The metric unit of fluid measure, equal to one cubic decimetre. Symbols: l, L, ℓ
- A measure of volume equivalent to a litre.