The most common and most likely answer to this crossword clue is the 12 letter word CONFIRMATION
I've seen this crossword clue in The Daily Telegraph
C
O
N
F
I
R
M
A
T
I
O
N
More clues leading to the result CONFIRMATION
Buddy explains!
The straight part
We know that "Sanction" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sanction = confirmation.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 CONFIRMATION mean?
CONFIRMATION noun- An official indicator that things will happen as planned.
- A verification that something is true or has happened.
- A ceremony of sealing and conscious acknowledgement of the faith in many Christian churches, typically around the ages of 14 to 18; considered a sacrament in some churches, including Catholicism, but not in most Protestant churches.