Crossword Genius

Old devil's got a couple of old stars forgetting their lines (2-3-2)

Ross

I believe the answer is:

so-and-so

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

'old' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

'devil's got a couple of old stars forgetting their lines' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for so-and-so that I've seen before include "Unnamed person" , "Unpleasant person - whose name has been forgotten!" , "Disliked person" , "Someone or other - unpleasant person!" , "Right little rascal!" .)

I've seen this clue in The Independent.
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