The most common and most likely answer to this crossword clue is the 8 letter word WRITHING
I've seen this crossword clue in The Sunday Telegraph
W
R
I
T
H
I
N
G
More clues leading to the result WRITHING
Buddy explains!

The straight part
We know that "Visibly in pain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case visibly in pain = writhing.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 WRITHING mean?
WRITHING verb- To twist, to wring (something).
- To contort (a part of the body).
- To twist or contort the body; to be distorted.
- To extort.
- A twisting struggle.
- Bended in twisting struggle.