What does the grammar "noun が noun、verb" mean?


In a game I am playing, I ran across the sentence,

> "ひらりと白い[欠片]{かけら}がひとひら、舞い込んできた。"

It appears that both 欠片 and ひとひら are nouns. After this, there is no particle
before the verb. What type of grammar structure is this?  
I found a translation online that suggests that this line means "Nimbly a
white fragment like a petal floated down," which makes sense, but I am not
sure how the sentence means this.




ひとひら is not a noun, but a number ([1]{ひと}) and a counter for a thin and flat
object like a petal ([片]{ひら} or [枚]{ひら}, usually written in kana). Thus, that
sentence has the same structure as 男が[1人]{ひとり}入ってきた, 髪が[10本]{じゅっぽん}抜けた, or
ケーキが[2]{ふた}つある.




ひとひら is not a noun, but a number ([1]{ひと}) and a counter for a thin and flat
object like a petal ([片]{ひら} or [枚]{ひら}, usually written in kana). Thus, that
sentence has the same structure as 男が[1人]{ひとり}入ってきた, 髪が[10本]{じゅっぽん}抜けた, or
ケーキが[2]{ふた}つある.

