The most common and most likely answer to this crossword clue is the 8 plus 5 letter word CALENDAR MONTH
I've seen this crossword clue in Independent
C
A
L
E
N
D
A
R
M
O
N
T
H
Buddy explains!
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.