— 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: