The most common and most likely answer to this crossword clue is the 8 letter word SCARRING
I've seen this crossword clue in The Daily Telegraph
S
C
A
R
R
I
N
G
More clues leading to the result SCARRING
Buddy explains!

The straight part
We know that "sign of injury" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sign of injury = scarring.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 SCARRING mean?
SCARRING verb- To mark the skin permanently.
- To form a scar.
- To affect deeply in a traumatic manner.
- A scar; a mark.