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.
Crossword Answer definitions
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.