The most common and most likely answer to this crossword clue is the 7 plus 6 letter word PERFECT NUMBER
I've seen this crossword clue in The Sunday Telegraph
P
E
R
F
E
C
T
N
U
M
B
E
R
More clues leading to the result PERFECT NUMBER
Buddy explains!
The straight part
We know that "Six, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case six, say = perfect number.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.