The most common and most likely answer to this crossword clue is the 8 letter word HANGNAIL
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Buddy explains!
The straight part
We know that "the cause of a sore finger?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the cause of a sore finger? = hangnail.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 HANGNAIL mean?
HANGNAIL noun- A loose, narrow strip of nail tissue protruding from the side edge and anchored near the base of a fingernail or toenail.
- A pointed upper corner of the toenail (often created by improperly trimming by rounding the corner) that, as the nail grows, presses into the flesh or protrudes so that it may catch (“hang”) on stockings or shoes.