The most common and most likely answer to this crossword clue is the 12 letter word UNFATHOMABLE
U
N
F
A
T
H
O
M
A
B
L
E
Buddy explains!
The straight part
We know that "bottomless" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bottomless = unfathomable.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 UNFATHOMABLE mean?
UNFATHOMABLE adjective- Impossible to fathom or understand.
- Difficult to penetrate.