I mean Tom has become an old fossil!

I believe the answer is:

ammonite

I'm a little stuck... Click here to teach me more about this clue!

'fossil' is the definition.
(I know that ammonite is a type of fossil)

'i mean tom has become an old' is the wordplay.
'i mean tom' can be anagrammed to 'AMMONITE'.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for ammonite that I've seen before include "A victim of Saul" , "Coiled fossil shell of extinct mollusc" , "Spiral fossil" , "shelled to extinction?" , "Common fossil" .)

I've seen this clue in The Irish Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.