The most common and most likely answer to this crossword clue is the 8 plus 4 letter word ACHILLES HEEL
I've seen this crossword clue in The Times
A
C
H
I
L
L
E
S
H
E
E
L
More clues leading to the result ACHILLES HEEL
- Ill in Chelsea, he developed a fatal weakness
- Suffering cutting pains, incline to show weakness
- Weak point of a cool fish she consumedÂ
- A cold fish accepting woman's weakness
- Weakness in a country novel the Spanish girl finally digested
- A cold fish admitting woman's his one weakness?
- Vulnerable spot targeted by Paris
- Shivering in water around bank is a sign of weakness
- A bit of a cold fish, accepting she is a weak point
- A cold fish entertains female showing weakness
Buddy explains!
The straight part
We know that "fatal weakness" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fatal weakness = achilles heel.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.