Mark of injury causing endless alarm (4)
I believe the answer is:
scar
'mark of injury' is the definition.
(I've seen this before)
'endless alarm' is the wordplay.
'endless' means to remove the last letter.
'alarm' becomes 'scare' (I've seen this before).
'scare' with its last letter removed is 'SCAR'.
'causing' is the link.
(Other definitions for scar that I've seen before include "Permanent mark left by a wound" , "Evidence of an old cut?" , "Craggy rock formation" , "Disfigure" , "Mark left on healed tissue" .)