There's not a lot of herbicide options especially if your clover is not at least a year or more old.
If the Curly Dock is in the seedling stage, Butyrac 200 (2,4-DB
kill it. It cost around $45.00 bucks a gallon. Raptor is another possibility but runs right around $550-600 a gallon
If the Curly Dock isn't real thick, then I would suggest using a hoe/shovel to dig out the plant, root and all. If it's very thick and over taking the clover, then you might want to consider spot spraying with a hot dose of glyphosate (roundUp) which is very economically priced right now ($13-30 a gallon)
Mowing the Dock about the time it's sending up a seed stalk but BEFORE it goes to seed is another option. It won't kill the Dock but it will slow it down to the point it's not competing as bad.