The most common and most likely answer to this crossword clue is the 6 letter word BULLET
I've seen this crossword clue in Independent
B
U
L
L
E
T
More clues leading to the result BULLET
- Point in report granted after size reduced
- Explosive device found in Kabul letterbomb
- Japanese train is shot
- Ox and swallow, but not a slug
- Slug in batter left as replacement for food initially
- Warn United backs after opening for blocked shot
- . Missile that kills uprising lamenting lives primarily taken by hazard
- Round tube made to cover two lengths
- Order book about enthralling origin of unusual slug
- Optimistic investor, extremely efficient, getting sack
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 = bullet.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 BULLET mean?
BULLET noun- A projectile, usually of metal, shot from a gun at high speed.
- An entire round of unfired ammunition for a firearm, including the projectile, the cartridge casing, the propellant charge, etc.
- Ammunition for a sling or slingshot which has been manufactured for such use.
- A printed symbol in the form of a solid circle, (•), often used in lieu of numbers for marking items in a list. (see also bulleted)
- A large scheduled repayment of the principal of a loan; a balloon payment.
- A rejection letter, as for employment, admission to a school or a competition.
- One year of prison time
- An ace (the playing card).
- Anything that is projected extremely fast.
- (in attributive use) Very fast (speedy).
- A plumb or sinker.
- (particularly in Northern Ireland) The heavy projectile thrown in a game of road bowling.
- A roughly bullet-shaped sweet consisting of a cylinder of liquorice covered in chocolate.
- A small ball.
- A cannonball.
- The fetlock of a horse.
- To draw attention to (text) by, or as if by, placing a graphic bullet in front of it.
- To speed, like a bullet.
- To make a shot, especially with great speed.
- A young or little bull; a male calf.