这是后置定语的一种用法,形容词组作定语须后置,其功能相当于一个定语从句.
这里形容词词组necessary to succeed 修饰elements ,相当于一个定语从句that is necessary to succeed
不是说它省略了that ,要说省略,也只能说它省略了that is.
多举几个例子：
They are the boys easiest to teach .他们是最容易教的孩子.（easiest to teach 相当于who are easiest to teach）
The basket full of some fruits belongs to the old woman .盛满各种水果的这个篮子是这位老妇人的.（full of some fruits 相当于which is full of some fruits ）
Those brave enough to take the course must be the top students in the grade .那些敢于选修这门课程的学生一定是本年级的尖子生.（brave enough 相当于who are brave enough）