The most common and most likely answer to this crossword clue is the 6 letter word UNTRUE
I've seen this crossword clue in Independent
U
N
T
R
U
E
More clues leading to the result UNTRUE
- Not the case guard used regularly to keep books in
- False regret follows quest, commonly
- Disloyal squad one left with regret
- Regret following unit when one’s gone wrong
- Don’t start to look for compassion. It’s not genuine
- Nut-cracking the French way isn’t right
- Sleeping around is out
- False starter making east turn twice
- Falsely disloyal
- Wrong ’un gets tortured now and then
Buddy explains!

The straight part
We know that "Sleeping around/out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sleeping around/out = untrue.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 UNTRUE mean?
UNTRUE adjective- False; not true.
- Not faithful or loyal.