The most common and most likely answer to this crossword clue is the 15 letter word LOCHNESSMONSTER
L
O
C
H
N
E
S
S
M
O
N
S
T
E
R
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.