The most common and most likely answer to this crossword clue is the 9 letter word SUFFERING
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The straight part
We know that "pain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pain = suffering.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
SUFFERING verb- To undergo hardship.
- To feel pain.
- To become worse.
- To endure, undergo.
- To allow.
- The condition of someone who suffers; a state of pain or distress.
- Experiencing pain.