The most common and most likely answer to this crossword clue is the 4 letter word KURU
K
U
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U
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The straight part
We know that "Brain disease" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case brain disease = kuru.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 KURU mean?
KURU noun- A chronic, progressive, fatal central nervous system disease found mainly among the Fore and neighboring peoples of New Guinea, caused by a prion that probably resembles the scrapie agent of sheep, transmissible to nonhuman primates, and believed to be transmitted by ritual cannibalism.