The most common and most likely answer to this crossword clue is the 5 plus 6 letter word FIFTH COLUMN
I've seen this crossword clue in Financial Times
F
I
F
T
H
C
O
L
U
M
N
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.