Pommy snack may tease old joke! (5,8)

I believe the answer is:

roast chestnut

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

'pommy snack' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'tease old joke' is the wordplay.
'tease old' becomes 'roast' (I can't explain this - if you can you should believe this answer much more).
'joke' becomes 'chestnut' (I've seen this before).
'roast'+'chestnut'='ROAST CHESTNUT'

'may' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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