Old Bill's formal nickname? (6)

I believe the answer is:

robert

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'old' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for robert that I've seen before include "Victorian premier" , "Boy's name" , "-- Browning, nineteenth century poet" , "- Edward Lee, US Confederate general" , "Butterfly" .)

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