The most common and most likely answer to this crossword clue is the 11 letter word PITCH-BLACK
I've seen this crossword clue in The Guardian
P
I
T
C
H
-
B
L
A
C
K
More clues leading to the result PITCH-BLACK
Buddy explains!
The straight part
We know that "Jet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jet = pitch-black.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 PITCH-BLACK mean?
PITCH-BLACK adjective- Very dark; without light.