The adjective clause in the sentence "They never fail who die for a noble cause" is “who die for a noble cause.”

- "They" is the subject of the sentence.
- "Never fail" is the main verb and predicate.
- "who die for a noble cause" is the adjective clause that provides additional information about the people referred to by "They." This clause describes the specific characteristics or actions of the people who never fail, namely, that they are the ones who die for a noble cause.

So, this adjective clause "who die for a noble cause" modifies and gives more information about the word "They" in the sentence.