Devil shows an earlier scar (3,4)
I believe the answer is:
old nick
'devil' is the definition.
(synonyms)
'an earlier scar' is the wordplay.
'an earlier' becomes 'old' (old can mean former or earlier).
'scar' becomes 'nick' (I am not sure about this - if you are sure you should believe this answer much more).
'old'+'nick'='OLD NICK'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for old nick that I've seen before include "reopened wound?" , "alternative to deep blue sea" , "Familiar name for the devil, ancient chap" , "'The prince of darkness (3,4)'" , "Newgate or Marshalsea?" .)