The most common and most likely answer to this crossword clue is the 12 letter word INFLAMMATION
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result INFLAMMATION
Buddy explains!
The straight part
We know that "a cause of pain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a cause of pain = inflammation.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 INFLAMMATION mean?
INFLAMMATION noun- The act of inflaming, kindling, or setting on fire.
- The state of being inflamed
- A condition of any part of the body, consisting of congestion of the blood vessels, with obstruction of the blood current, and growth of morbid tissue. It is manifested outwardly by redness and swelling, attended with heat and pain.
- Violent excitement