The most common and most likely answer to this crossword clue is the 10 letter word CROSSBONES
I've seen this crossword clue in The Guardian
C
R
O
S
S
B
O
N
E
S
More clues leading to the result CROSSBONES
- The mark of pirate mix remains
- Christian symbol remains a reminder of mortality
- Angry doctor often portrayed with skull
- Old women seizing corrupt boss is a bit of a warning sign
- Image beneath skull generally frustrated doctor
- Device supporting skull likely to snap, initially bending nose out of shape
- Menacing symbol vexed British figures
Buddy explains!

The straight part
We know that "a reminder of mortality" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a reminder of mortality = crossbones.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 CROSSBONES mean?
CROSSBONES noun- A representation of two bones crossed diagonally, especially as a symbol of danger or death