The most common and most likely answer to this crossword clue is the 4 letter word BUMP
I've seen this crossword clue in The Guardian
B
U
M
P
More clues leading to the result BUMP
Buddy explains!

The straight part
We know that "jar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jar = bump.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 BUMP mean?
BUMP noun- A light blow or jolting collision.
- The sound of such a collision.
- A protuberance on a level surface.
- A swelling on the skin caused by illness or injury.
- One of the protuberances on the cranium which, in phrenology, are associated with distinct faculties or affections of the mind. Also (metonymically) the faculty itself
- The point, in a race in which boats are spaced apart at the start, at which a boat begins to overtake the boat ahead.
- The swollen abdomen of a pregnant woman.
- A post in an Internet forum thread made in order to raise the thread's profile by returning it to the top of the list of active threads.
- A temporary increase in a quantity, as shown in a graph.
- A dose of a drug such as ketamine or cocaine, when snorted recreationally.
- The noise made by the bittern; a boom.
- (preceded by definite article) A disco dance in which partners rhythmically bump each other's hips together.
- In skipping, a single jump over two consecutive turns of the rope.
- A coarse cotton fabric.
- A training match for a fighting dog.
- The jaw of either of the middle pockets.
- Music, especially played over speakers at loud volume with strong bass frequency response.
- To knock against or run into with a jolt.
- To move up or down by a step; displace.
- To post in an Internet forum thread in order to raise the thread's profile by returning it to the top of the list of active threads.
- (of a superheated liquid) To suddenly boil, causing movement of the vessel and loss of liquid.
- To move (a booked passenger) to a later flight because of earlier delays or cancellations.
- To move the time of (a scheduled event).
- To pick (a lock) with a repeated striking motion that dislodges the pins.
- To make a loud, heavy, or hollow noise; to boom.
- To spread out material so as to fill any desired number of pages.