No, it's right. "so silly a fancy" = "such a silly fancy". A "fancy" is a notion -- usually one that's not based very solidly in reality.
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TanitMy question is: are there any other words for which this particular word order (X + adjective + a/an + noun) applies?
More specifically, can I use it with quite, really and rather?
He's quite/really/rather talented a writer. (?)
Tanitare there any other words for which this particular word order (X + adjective + a/an + noun) applies?