The most common and most likely answer to this crossword clue is the 8 letter word ETIOLOGY
I've seen this crossword clue in Independent
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More clues leading to the result ETIOLOGY
Buddy explains!

The straight part
We know that "US disease study" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case us disease study = etiology.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 ETIOLOGY mean?
ETIOLOGY noun- The establishment of a cause, origin, or reason for something.
- The study of causes or causation.
- The study or investigation of the causes of disease; a scientific explanation for the origin of a disease.