The most common and most likely answer to this crossword clue is the 10 plus 8 letter word CONSPIRACY THEORIST
I've seen this crossword clue in The Guardian
C
O
N
S
P
I
R
A
C
Y
T
H
E
O
R
I
S
T
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.