The most common and most likely answer to this crossword clue is the 12 letter word OSTEOPOROSIS
I've seen this crossword clue in Financial Times
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More clues leading to the result OSTEOPOROSIS
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The straight part
We know that "Nasty condition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nasty condition = osteoporosis.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.
Crossword Answer definitions
OSTEOPOROSIS noun- A disease, occurring especially in women following menopause, in which the bones become extremely porous and are subject to fracture.