essay是论文，不是散文。

a designer's opinion written in fabric on the body 这是独立主格结构，作句子的原因状语。即解释为什么说时装表演就像一篇论文：因为设计师的观点是用身上的布料书写而成的。

in a given time和独立主格结构都是句子状语。这个介词短语可以放在句首。这个句子可以有以下几种变体：

In a given time period, a show is like an essay, a designer's opinion written in fabric on the body.

A show, a designer's opinion written in fabric on the body. is like an essay, in a given time period.

A designer's opinion written in fabric on the body, a show is like an essay, in a given time period

以上三个句子中，a designer's opinion written in fabric on the body均为独立主格结构作状语。