Sign of an old injury (4)
I believe the answer is:
'sign of an old injury' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for scar that I've seen before include "Bare rocky cliff - injury" , "Effect of unpleasant experience" , "fall could possibly be the reason for one" , "Mark of a wound remaining on the skin" , "Mark left by the healing of injured tissue" .)