Can a plural subject be followed by a complement beginning “a…”?

Can anyone kindly explain why this sentence is correct?

Complex musical numbers are a defining characteristic of most Italian
films.

Answer

In a comment, John Lawler wrote:

A defining characteristic of Italian films is not a subject; it is a noun phrase that is the predicate of the sentence, following the be form that is the required auxiliary for predicate nouns. He is an idiot, She is a genius, They are the ones, all use predicate noun phrases, following be, using whatever articles those noun phrases use. They are not subjects; they are predicates and they don’t have to agree with anything.

Attribution
Source : Link , Question Author : HeyDoeFarm , Answer Author : tchrist

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