An open letter to old people (6)

I believe the answer is:

franks

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'old people' is the definition.
Although both the answer and definition are plural nouns, I cannot understand how they can define each other.

'an open letter' is the wordplay.
'an open' becomes 'frank' (I've seen this in another clue).
'letter' becomes 's' ().
'frank'+'s'='FRANKS'

'to' is the link.

Can you help me to learn more?

(Another definition for franks that I've seen is " old European people".)

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