The most common and most likely answer to this crossword clue is the 3 letter word LIS
I've seen this crossword clue in The Times
L
I
S
Buddy explains!

The straight part
We know that "heraldic device" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case heraldic device = lis.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 LIS mean?
LIS noun- Fleur-de-lis
- The Chinese mile, a traditional unit of distance equal to 1500 chis or 150 zhangs, now standardized as a half-kilometer (500 meters).
- The Korean mile, a traditional unit of distance equivalent to about 393 m.