The most common and most likely answer to this crossword clue is the 7 letter word ABYSMAL
I've seen this crossword clue in The Guardian
A
B
Y
S
M
A
L
More clues leading to the result ABYSMAL
- Dreadful article: Times hits back
- Terrible Barnaby's maladjusted to some extent
- Dreadful cooked lamb, say
- Very poor baby, small, uncovered
- Stewed lamb, say, is awful
- Horrible baby’s malodorous sample
- A little baby, small and woeful
- Very bad baby’s malodorous clothes
- Lamb, say, is perhaps unfathomable
- Lousy letters from Lamb, say
Buddy explains!

The straight part
We know that "unfathomable" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unfathomable = abysmal.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 ABYSMAL mean?
ABYSMAL adjective- Pertaining to, or resembling an abyss.
- Extremely bad; terrible.