The most common and most likely answer to this crossword clue is the 9 plus 2 plus 6 letter word DISCIPLES OF CHRIST
I've seen this crossword clue in The Times
D
I
S
C
I
P
L
E
S
O
F
C
H
R
I
S
T
Buddy explains!
The straight part
We know that "Say John and Paul" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case say john and paul = disciples of christ.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.