The most common and most likely answer to this crossword clue is the 13 letter word INSUBSTANTIAL
I've seen this crossword clue in The Daily Telegraph
I
N
S
U
B
S
T
A
N
T
I
A
L
More clues leading to the result INSUBSTANTIAL
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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.
What does INSUBSTANTIAL mean?
INSUBSTANTIAL adjective- Lacking substance; not real or strong.