grammatical number - S-V agreement: It is not clear what is/are meant by A and B

In the following sentence, the verb “are” strikes me as odd.

In paragraph 6, it is not clear what are meant by “the front unit” and “the central element”.

It seems that “. . . it is not clear what is meant by . . .” would be more natural.

However, thinking more about it and flipping the sentence around yields something like:

“the front unit” and “the central element” mean what?

So, it seems that logically the verb should actually be “are”?

What is going on here? Is the plural correct (are)? Or is there actually some other subject in the original sentence, and therefore “is”.

Answer

You are correct: it should be is. The writer perhaps mistakenly treated by “the front unit” and “the central element” as the subject, which in fact is singular what. The by phrase is an adverbial constituent of instrument or similar.

Now it could be argued that what should be plural in this clause, which is in theory possible. However, it could just as well be analysed as singular: the clause could be considered elliptical, where parts have been omitted for brevity, as it is normally done:

In paragraph 6, it is not clear what is meant by x and [what is meant] by y.

Moreover, the phrase what is meant is almost universally used in the singular and could be said to be idiomatically fixed. The plural sounds odd to my ears.