Is “a becoming humility” a correct use? [duplicate]

Though this may seem esoteric, often the use of a adjectival modifier can render an uncountable into a countable, and this is but one example; you can test this proposition by replacing the adjective "becoming" with synonyms, near synonyms or antonyms, and noting that the resulting sentences still work with the "a" in place.

"She accepted the award with a charming humility"

"She accepted the award with a strange humility"

"She accepted the award with an atypical humility"

"She accepted the award with a shocking arrogance"

"She accepted the award with an unbecoming hubris"

They all work just fine.

Yes, uncountable nouns may be preceded by an adjective plus the article a/an in certain circumstances:

"These circumstances are when you are qualifying or limiting the noun’s meaning in some way." MacMillan Dictionary

becoming qualifies the humility.

• Truth lies behind this brouhaha.
• A greater truth lies behind all this brouhaha.
• Honesty can cure many ills.
• An unusual honesty characterized their relationship.

indefinite article with uncountable nouns

Yes, it is correct. See the explanation of Cambridge dictionary A | meaning in the Cambridge English Dictionary:

used before some uncountable nouns when you want to limit their meaning in some way, such as when describing them more completely or referring to one example of them:

I only have a limited knowledge of Spanish.

He has a great love of music.

There was a fierceness in her voice.