The most common and most likely answer to this crossword clue is the 4 letter word STOP
I've seen this crossword clue in The Guardian
S
T
O
P
Buddy explains!
The straight part
We know that "Pack up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pack up = stop.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 STOP mean?
STOP noun- A (usually marked) place where buses, trams or trains halt to let passengers get on and off, usually smaller than a station.
- An action of stopping; interruption of travel.
- That which stops, impedes, or obstructs; an obstacle; an impediment.
- A device intended to block the path of a moving object
- A consonant sound in which the passage of air through the mouth is temporarily blocked by the lips, tongue, or glottis.
- A symbol used for purposes of punctuation and representing a pause or separating clauses, particularly a full stop, comma, colon or semicolon.
- A knob or pin used to regulate the flow of air in an organ.
- One of the vent-holes in a wind instrument, or the place on the wire of a stringed instrument, by the stopping or pressing of which certain notes are produced.
- A very short shot which touches the ground close behind the net and is intended to bounce as little as possible.
- The depression in a dog’s face between the skull and the nasal bones.
- A part of a photographic system that reduces the amount of light.
- A unit of exposure corresponding to a doubling of the brightness of an image.
- An f-stop.
- The diaphragm used in optical instruments to cut off the marginal portions of a beam of light passing through lenses.
- A coup d'arret, or stop thrust.
- To cease moving.
- To not continue.
- To cause (something) to cease moving or progressing.
- To cease; to no longer continue (doing something).
- To cause (something) to come to an end.
- To close or block an opening.
- (often with "up" or "down") To adjust the aperture of a camera lens.
- To stay; to spend a short time; to reside or tarry temporarily.
- To regulate the sounds of (musical strings, etc.) by pressing them against the fingerboard with the finger, or otherwise shortening the vibrating part.
- To punctuate.
- To make fast; to stopper.
- A small well-bucket; a milk-pail.
- Being or relating to the squark that is the superpartner of a top quark.