Crossword Genius

Old Bill, or Robert (5)

Ross

I believe the answer is:

bobby

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

'old bill' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'robert' is the wordplay.
I cannot quite understand how this works, but
'robert' could be 'bob' (short for Robert) and 'bob' is present in the answer.
The remaining letters 'by' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

'or' acts as a link.

Can you help me to learn more?

(Other definitions for bobby that I've seen before include "Cop (slang)" , "beat man" , "Policeman (sl.)" , "(Local) policeman" , "Rob" .)

I've seen this clue in the Evening Standard.
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