Plan One not working? Not quite (4)
I believe the answer is:
idea
'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" .)