The most common and most likely answer to this crossword clue is the 6 plus 7 letter word LETTER PERFECT
I've seen this crossword clue in The Daily Telegraph
L
E
T
T
E
R
P
E
R
F
E
C
T
Buddy explains!

The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does LETTER PERFECT mean?
LETTER PERFECT adjective- Exactly correct, correct in every letter.