The most common and most likely answer to this crossword clue is the 8 letter word STINGRAY
I've seen this crossword clue in The Times
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More clues leading to the result STINGRAY
- Artist with tight clothing threatening swimmer
- Fish going off course with beam of light shifting eastward
- Saint at home with elegist? Something fishy here
- Mean to cross river with a fish
- Servant’s opening can of silvery American fish
- Penny-pinching painter taken in, one to be wary of
- Ratings cooked unknown fish
- Artist preserved by mean fish!
- Mean to eat a lot of raw fish
- Fish eggs turned grey, reportedly
Buddy explains!

The straight part
We know that "threatening swimmer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case threatening swimmer = stingray.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 STINGRAY mean?
STINGRAY noun- Any of various large, venomous rays, of the orders Rajiformes and Myliobatiformes, having a barbed, whiplike tail.
- A device that simulates a cell tower, used to intercept cell phone communications.