Tafsir of Al-Anbiya' 21:70

"And they intended against him a plot..."

That is, a great scheme to harm him and overcome him.

"...so We made them the greatest losers."

Meaning: the greatest of all losers, for their effort to extinguish the light of truth—both in speech and action—returned as a decisive proof that he, peace be upon him, was upon the truth and they were upon falsehood, and it necessitated the elevation of his rank, peace be upon him, and their deserving the most severe punishment.

It is also said: He made them the greatest losers in the sense that He, Glory be to Him, empowered against them the most contemptible and weakest of His creatures, which is the gnat; it ate their flesh and drank their blood. He also empowered a gnat against Nimrod, which continued to torment him until he died—may the curse of Allah be upon him.

The interpretation to be relied upon is the first one.