The most common and most likely answer to this crossword clue is the 6 letter word WEDGED
I've seen this crossword clue in Financial Times
W
E
D
G
E
D
Buddy explains!

The straight part
We know that "jammed" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jammed = wedged.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 WEDGED mean?
WEDGED verb- To support or secure using a wedge.
- To force into a narrow gap.
- To work wet clay by cutting or kneading for the purpose of homogenizing the mass and expelling air bubbles.
- Of a computer program or system: to get stuck in an unresponsive state.
- To cleave with a wedge.
- To force or drive with a wedge.
- To shape into a wedge.
- Cuneiform or wedge-shaped
- Stuck; incapable of proceeding without help, though not having crashed.