\(\sqrt{\sqrt{3}-\sqrt{1-\sqrt{21-12\sqrt{3}}}}\)
\(=\sqrt{\sqrt{3}-\sqrt{1-\sqrt{\left(2\sqrt{3}\right)^2-2\cdot2\sqrt{3}\cdot3+3^2}}}\)
\(=\sqrt{\sqrt{3}-\sqrt{1-\sqrt{\left(2\sqrt{3}-3\right)^2}}}\)
\(=\sqrt{\sqrt{3}-\sqrt{1-2\sqrt{3}+3}}\)
\(=\sqrt{\sqrt{3}-\sqrt{\left(\sqrt{3}-1\right)^2}}\)
\(=\sqrt{\sqrt{3}-\sqrt{3}+1}\)
\(=\sqrt{1}=1\)