I have a clear idea of what I want vs I have a clear idea about what I want
1 I have a clear idea of what I want
2 I have a clear idea about what I want

Both sentences are correct and they mean the same thing. Am I right?
So "of" here is still the possessive usage?

Question

I have a clear idea of what I want vs I have a clear idea about what I want
1 I have a clear idea of what I want 
2 I have a clear idea about what I want

Both sentences are correct and they mean the same thing. Am I right?
So "of" here is still the possessive usage?

Answer

You are right. I think "of" is possessive here, because we can think of it this way.

I have an idea.
What kind of idea?
A "what I want" idea.
An idea of what I want.

Answer

Although both are correct, the second doesn't sound quite so natural to me.  I would always use 'of' in this context.  Not sure if that's just my style, or if others would agree.