The most common and most likely answer to this crossword clue is the 4 letter word SWAT
I've seen this crossword clue in The Guardian
S
W
A
T
More clues leading to the result SWAT
- Didn't stand around waiting initially for strike
- To kill flies is heartless hard work
- Strike that wasp periodically returning
- Squash mug
- Hit heartless drip
- Kill flies? It sounds like hard work
- Smack overly diligent student by one’s ear?
- Trap wasp — oddly, rotating fly-killer could do this
- Heads of senior Whigs and Tories hit sharply
- American police team positioned to apprehend women
Buddy explains!

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 SWAT mean?
SWAT noun- A hard stroke, hit or blow, e.g., as part of a spanking.
- Alternate spelling of swot: vigorous study at an educational institution.
- To beat off, as insects; to bat, strike, or hit.
- To illegitimately provoke a SWAT assault upon (someone).