前面一句完整的句子,逗号后 a fact/a belief.

也可以试试这样去理解这个句子
先不理 that leads to many ...这部分
In Death of a Salesman Willy Loman mistakenly believes that his sons have
no flaws,a belief.
可以还原为
In Death of a Salesman Willy Loman mistakenly believes that his sons have
no flaws,which is a belief.
这亦即是有一主句(In Death of a Salesman Willy Loman mistakenly believes
that his sons have no flaws)
以及一个用which而成的从句( which is a belief)
不过
英文的写作每每讲求精简
也就是删略一些不必要的字
或将句子改为短语
而 which 这个从句可以删减
其中一个方法是除去 which 和 is
剩下 a belief
由於 a belief 是名词结构
所以便是同位语( appositive)
在原文是指’Willy Loman mistakenly believes that his sons have no flaws
‘整句的意思
也许你会质疑
单就In Death of a Salesman Willy Loman mistakenly believes that his sons
have no flaws,a belief 这句来说
a belief的出现很不自然
是的
但如果稍为增加一点意思
便不会有这样的感觉
列如
In Death of a Salesman Willy Loman mistakenly believes that his sons have
no flaws,a fatal belief
最后补充that 从句
that leads to many problems for the entire family
是用that 而做成的 关系从句 (relative clause)
that 代表了 belief

倒装句

这种用法很常见。a belief/fact是同位语，往往用于对前面句子或单词的进一步补充说明，或解释其含义。belief/factt等名词（词组）后面往往会有后置定语，充当后置定语的可以是短语，也可以是从句，以便把意思表达完整。
～～英语教师团～～

同位语是了。不要解释为fact/belief后面有后置定语，定语就是定语怎么又是同位语呢？