The most common and most likely answer to this crossword clue is the 5 letter word ALAMO
I've seen this crossword clue in The Times
A
L
A
M
O
More clues leading to the result ALAMO
- Nearly all with a second mission
- Leader of armed mob almost seizing city in battle
- Last pair abandoning fashionable, ill-fated mission
- Some trial, a month in siege site
- Mission appearing in article by the French medical officer
- Stylish shot of duke and English battle scene
- Recalling doctor's old mission
- Sadly not allowed to complete doctor’s mission
- Mission accomplished — like another mission — on all fronts!
- Part of total amount needed for Texas mission
Buddy explains!

The straight part
We know that "ill-fated mission" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ill-fated mission = alamo.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.