The most common and most likely answer to this crossword clue is the 4 plus 3 letter word DEAD LEG
I've seen this crossword clue in Financial Times
D
E
A
D
L
E
G
Buddy explains!
The straight part
We know that "impact injury" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case impact injury = dead leg.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 DEAD LEG mean?
DEAD LEG noun- A blow to the upper thigh, crushing the muscle against the bone and crushing the nerve cluster next to the quadriceps. Also, the resulting injury.
- A lazy or weak person.
- An isolated section of pipeline that does not usually carry a flow.
- A movement in which the hips and knees are kept straight and stiff and the ankle is flexed.
- (shipping) A vehicle that is traveling on a leg of its route in which it does not carry any cargo.