Crossword Genius

Plan One not working? Not quite (4)

Ross

I believe the answer is:

idea

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

'plan' is the definition.
(both can mean an intention)

'one not working? not quite' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
The remaining letters 'dea' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for idea that I've seen before include "Theory" , "Notion, concept" , "Sudden inspiration?" , "It comes to mind" , "Creative spark" .)

I've seen this clue in The Times.
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