How's it goin'?
Oh, hangin' in.
Is "hangin' in" okay as answer to the question?
"Hanging in there" means "I'm still trying to keep going."
Okay, I hear your answers.
But what if someone just says "Hangin' in" in response to the question, "How's it goin'?"
I think it would be far more common to hear "Hangin'" than "Hangin' in," but if I ever did hear "hangin' in," I would automatically translate it as "Hanging in there."
"Chillin'" is an example of a use by someone too cool to say the entire expression (chilling out), so I would interpret "hangin'" or "hangin' in" in the same way -- as way cool.
To me, it's an example of how language is unpredictable. I had not heard "hangin' in" before. When I hear such things, I sometimes think about how one could respond to an ELL asking a question about "hangin'".
I wouldn't be one to simply say "it's wrong". You must say "there". I'd rather hesitate and give a question some thought. Sometimes ELLs would like to think that it's either right or wrong, and the one's that are ELTs where they come from are trained to say it's "informal". I don't buy "informal" as an explanation, though many such expressions are clearly informal.