The most common and most likely answer to this crossword clue is the 6 letter word NOTION
I've seen this crossword clue in The Sunday Telegraph
N
O
T
I
O
N
More clues leading to the result NOTION
- Thought anyone but myself available
- I will interrupt unthinkable idea
- One taken in by unacceptable idea
- I must block unacceptable idea
- Fancy sex turning up in middle of day!
- Belief unacceptable across India
- Unacceptable to claim inventor’s first idea
- Idea‘s impracticable without one
- Tiny slice of tomato in fried onion: there’s an idea
- Unacceptable to hold one idea
Buddy explains!

The straight part
We know that "idea" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case idea = notion.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NOTION mean?
NOTION noun- Mental apprehension of whatever may be known, thought, or imagined; idea, concept.
- A sentiment; an opinion.
- Sense; mind.
- An invention; an ingenious device; a knickknack.
- Any small article used in sewing and haberdashery, either for attachment to garments or as a tool, such as a button, zipper, or thimble.
- Inclination; intention; disposition.