Devil showing familiar blemish (3,4)
I believe the answer is:
old nick
'devil' is the definition.
(synonyms)
'familiar blemish' is the wordplay.
'familiar' becomes 'old' (old can mean familiar or traditional).
'blemish' becomes 'nick' (nick is a kind of blemish).
'old'+'nick'='OLD NICK'
'showing' acts as a link.
(Other definitions for old nick that I've seen before include "spirit of evil" , "Familiar name for the devil, ancient chap" , "Mephisto" , "'The prince of darkness (3,4)'" , "The devil, Lucifer" .)