Off about one? That's the idea (6)
I believe the answer is:
notion
'idea' is the definition.
(I know that idea can be written as notion)
'off about one? that's the' is the wordplay.
I cannot really see how this works, but
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is located in the answer.
'one?' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for notion that I've seen before include "Whim or fancy" , "Vague understanding" , "Vague or fanciful idea" , "Idea, belief" , "Idea or fancy" .)