The most common and most likely answer to this crossword clue is the 8 letter word HOARIEST
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More clues leading to the result HOARIEST
Buddy explains!

The straight part
We know that "Most grizzled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case most grizzled = hoariest.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 HOARIEST mean?
HOARIEST adjective- White, whitish, or greyish-white.
- White or grey with age.
- Of a pale silvery grey.
- Covered with short, dense, greyish white hairs; canescent.
- Remote in time past.
- Moldy; mossy; musty.
- Old or old-fashioned.