The most common and most likely answer to this crossword clue is the 3 plus 8 plus 3 letter word ALL HALLOWS’ DAY
I've seen this crossword clue in The Guardian
A
L
L
H
A
L
L
O
W
S
â
€
™
D
A
Y
Buddy explains!
The straight part
We know that "1/11" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 1/11 = all hallows’ day.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.