Sign of old injury, frightening cut (4)
I believe the answer is:
scar
'sign of old injury' is the definition.
(I've seen this before)
'frightening cut' is the wordplay.
'frightening' becomes 'scary' (I've seen this before).
'cut' means to remove the last letter.
'scary' with its final letter removed is 'SCAR'.
(Other definitions for scar that I've seen before include "Permanent mark left by a wound" , "Evidence of an old injury" , "Sign of wound" , "fall could possibly be the reason for one" , "Disfigure" .)