The most common and most likely answer to this crossword clue is the 3 letter word BED
I've seen this crossword clue in Independent
B
E
D
More clues leading to the result BED
- Education graduate’s place to rest
- Open-plan residence adapted to big modernist
- Qualified teacher gets the sack
- Lying as the solution to one sort of problem?
- County with it can prepare compact living quarters
- / 12d Furniture and computers can improve the appearance of cramped accommodation
- Matrimonial duties? Adam falls short
- Flat for those not wishing to move when they retire
- Piece of camping kit to cause amusement
Buddy explains!

The straight part
We know that "sack" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sack = bed.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 BED mean?
BED noun- A piece of furniture, usually flat and soft, on which to rest or sleep.
- A place, or flat surface or layer, on which something else rests or is laid.
- (heading) A layer or surface.
- Senses relating to a bed as a place for resting or sleeping.
- Senses relating to a bed as a place or layer on which something else rests or is laid.