***** NOT A TEACHER *****
You are a fantastic student. I am sure that every teacher in the States would give you an A+ (the highest mark
possible) for the great job that you did in parsing that sentence.
You are 100% right: "something useful" is an ellipsis [omitting some words] of "something that is useful."
Here is a wonderful example from one of the greatest grammarians in the history of English (only my opinion,
"Something [which is] new, nothing [which is] extraordinary, everything [which is] English."
Another one of his examples:
"Let Jenny marry somebody [who is] rich."
The rule: "A single adjective frequently stands after an indefinite pronoun."
That great scholar was Professor George Oliver Curme, His 1931 two-volume masterpiece is entitled A Grammar of the English Language. Refer to pages 64 - 65 of the second volume.
1. You used "that." He used "which." Don't worry. In this case, it doesn't matter which one you use.
2. It is absolutely NOT important, but you accidentally forgot to parse the preposition "about."
Student or Learner