An article explanation

[SARDORBEK]

She dreamt of a future where she could spend more time painting.

Generally, we use the definite article with the noun when there is a defining phrase
or relative clause after it, don’t we?

In this case, instead of the indefinite article, can we use the definite one from a strict grammatical point of view? Please could YOU explain the use of “a” in this case?
I was asked the question, but I could not answer.
I need your help...