Crash badly, sustaining extremely traumatic wound (7)
I believe the answer is:
scratch
'wound' is the definition.
(scratch is a kind of wound)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for scratch that I've seen before include "claw" , "Slight cut" , "Score, mark; cancel" , "Withdraw - abrasion" , "Insignificant injury" .)