The most common and most likely answer to this crossword clue is the 8 plus 1 letter word GUMBOIL –
I've seen this crossword clue in The Daily Telegraph
G
U
M
B
O
I
L
Â
â
€
“
Buddy explains!
The straight part
We know that "pain in the mouth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pain in the mouth = gumboil –.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 GUMBOIL – mean?
GUMBOIL – noun- A small suppurating inflamed spot on the gum.