The most common and most likely answer to this crossword clue is the 7 letter word MENACED
I've seen this crossword clue in The Guardian
M
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C
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More clues leading to the result MENACED
- Threatened fix involves good service
- Threatened staff with a first-class degree
- Expert in fix, under threat
- Repair on outside of a church is put at risk
- Threatened people defeated by first shot
- Hands account to newspaper boss when threatened
- Threatened to effect a repair to cover crack
- Threatened males got perfect scores
- Threatened players sailed through
- Threatened fellows performed very well
Buddy explains!

The straight part
We know that "under threat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case under threat = menaced.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 MENACED mean?
MENACED verb- To make threats against (someone); to intimidate.
- To threaten (an evil to be inflicted).
- To endanger (someone or something); to imperil or jeopardize.
- Subject to menaces; threatened