The most common and most likely answer to this crossword clue is the 8 letter word CONSERVE
I've seen this crossword clue in The Sunday Telegraph
C
O
N
S
E
R
V
E
More clues leading to the result CONSERVE
- Prisoner to hand around jam
- Scone removed from wrapping, provide jam
- Prisoner to wait for nurse
- Politician with minister to protect
- Devastated party: minister to keep back
- After party, spoon out jam
- Criminals do time sharing small guard
- Make sparing use of jam
- Study to be a fighter, and keep in good shape
- In jam, Tories veer off
Buddy explains!

The straight part
We know that "jam" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case jam = conserve.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 CONSERVE mean?
CONSERVE noun- Wilderness where human development is prohibited.
- A jam or thick syrup made from fruit.
- A medicinal confection made of freshly gathered vegetable substances mixed with finely powdered refined sugar.
- A conservatory.
- To save for later use, sometimes by the use of a preservative.
- To protect an environment.
- To remain unchanged during a process