The most common and most likely answer to this crossword clue is the 12 letter word WORD-PERFECT
I've seen this crossword clue in The Times
W
O
R
D
-
P
E
R
F
E
C
T
Buddy explains!
The straight part
We know that "faithful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case faithful = word-perfect.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 WORD-PERFECT mean?
WORD-PERFECT adjective- Correct in every word.
- Having memorized one's lines perfectly.