The most common and most likely answer to this crossword clue is the 4 letter word LULL
I've seen this crossword clue in The Daily Telegraph
L
U
L
L
More clues leading to the result LULL
- Respite during cellulite reversal
- Quiet university, three students seen around
- Calm group of students in uniform? On the contrary
- Three students welcoming university break
- Learner repeatedly going outside university for quiet time
- University in 50s, calm period
- Quiet university students after end of oral
- Three students seen around university in break
- Left for dead in sleepy calm
- Singer finished early, left silence
Buddy explains!

The straight part
We know that "Quiet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case quiet = lull.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 LULL mean?
LULL noun- A period of rest or soothing.
- A period of reduced activity; a respite
- A period without waves or wind.
- An extended pause between sets of waves.
- To cause to rest by soothing influences; to compose; to calm
- To become gradually calm; to subside; to cease or abate.