They process leather for old sixpences (7)

I believe the answer is:

tanners

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

'sixpences' is the definition.
(I know this)

'they process leather for old' is the wordplay.
I cannot quite understand how this works, but
'process' could be 'tan' (I've seen this before) and 'tan' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

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.