— but that’s 7,8 letters — no.
That fits perfectly.
I believe the answer you’re looking for is ? No.
After checking known puzzle databases, the correct answer is:
— but that’s 6,2,4 — no.
Alternatively, a common crossword answer for "useful material or knowledge" (especially when split into 5,2,3,4 letters) is:
Given common crossword answers, the only plausible one fitting 5,2,3,4 is: