Mark producing endless alarm (4)
I believe the answer is:
scar
'mark' is the definition.
(mark left by an injury)
'endless alarm' is the wordplay.
'endless' means to remove the last letter.
'alarm' becomes 'scare' (I've seen this in another clue).
'scare' with its last letter taken off is 'SCAR'.
'producing' acts as a link.
(Other definitions for scar that I've seen before include "Evidence of an old injury" , "Healed-cut mark" , "Evidence of a past injury?" , "Nasty mark" , "Remnant of wound" .)