The most common and most likely answer to this crossword clue is the 8 letter word BURSITIS
I've seen this crossword clue in The Daily Telegraph
B
U
R
S
I
T
I
S
Buddy explains!

The straight part
We know that "Painful inflammation" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case painful inflammation = bursitis.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 BURSITIS mean?
BURSITIS noun- An inflammation of a bursa, most common in the shoulder, elbow or knee