Unfortunately the Japanese language lacks this feature. Even if you don't like it, you have to rephrase it. For example, to say the least risky agreement,
- もっとも危険性が低い合意
- いちばんましな合意
- もっとも安全な合意
- [?]もっとも危険でない合意 (clumsy and ambiguous)
- [x]もっとも危険ではない合意 (this usually means "not the most dangerous", i.e., not necessarily the safest but at least not the worst)
As you can see, using ない can result in ambiguous expressions and should be avoided IMHO.
The same thing can be said for less + (adjective). For example less important clue would be:
- 重要性がより低い手がかり
- 比較的重要でない手がかり
- それほど重要でない手がかり
- 相対的にどうでもよい手がかり
- ○○ほどには重要でない手がかり
FWIW, this page describes this English construction (less/least + (adjective)) for Japanese people who learn English. (Note that some Japanese translations there are ambiguous)