The most common and most likely answer to this crossword clue is the 12 letter word SOMNAMBULIST
S
O
M
N
A
M
B
U
L
I
S
T
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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.