The most common and most likely answer to this crossword clue is the 4 letter word LAIR
I've seen this crossword clue in Independent
L
A
I
R
More clues leading to the result LAIR
- The French going to one river hideaway
- Den left with strain
- Scottish landowner’s unfinished retreat
- Den left impression
- Place for hiding money in recession
- Regularly visit sleazier den
- Hide style, not showing head
- PM, less bold, in retreat
- Former prime minister – no leader – in retreat
- Animal home left impression
Buddy explains!

The straight part
We know that "den" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case den = lair.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 LAIR mean?
LAIR noun- A place inhabited by a wild animal, often a cave or a hole in the ground.
- A shed or shelter for domestic animals.
- A place inhabited by a criminal or criminals, a superhero or a supervillain; a refuge, retreat, haven or hideaway.
- A bed or resting place.
- A grave; a cemetery plot.
- To rest; to dwell.
- To lay down.
- To bury.
- A bog; a mire.
- To mire.
- To become mired.
- A person who dresses in a showy but tasteless manner and behaves in a vulgar and conceited way; a show-off.