The most common and most likely answer to this crossword clue is the 8 letter word AMMONITE
I've seen this crossword clue in Independent
A
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The straight part
We know that "a fossil" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a fossil = ammonite.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 AMMONITE mean?
AMMONITE noun- Any of an extinct group of cephalopods of the subclass Ammonoidea; a fossil shell of such an animal.
- An explosive prepared from a mixture of TNT and ammonium nitrate; a form of amatol, popular in Eastern Europe and China.