How do you say something like that?
"There's a thing for which I don't know what it is" "There's a thing where I don't know what it is" "There's a thing that I don't know what is"
or (the one which I hear people say a lot but sounds awkward:) "There's a thing that/which I don't know what it is"?
To be honest they all sound awkward to me to varying degrees
This is a great question, and it led me down a bit of a rabbit hole. This kind of clause is called a Gapless Relative Clause. The sentence could be written as you have it, or with "I don't know what it is" - the "it" is called the Resumptive Pronoun which are "common in spoken English but are officially ungrammatical".
The Wikipedia article has a similar example:
Here's another great article I found which sums it up well:
"Resumptives are non-standard, but in such cases they're much better than their gapped counterparts, which people usually find incomprehensible, or at least very hard to comprehend."
So basically, your original sentence is "unacceptable"/"incomprehensible", but adding "it" would be grammatically incorrect but easier to understand. Best bet is probably to totally rephrase the sentence as others have suggested.
deleted by creator