Pilot, As a general rule spraying while the plant is actively growing is best. This increases the plants uptake of the chemical. There are exceptions with pre-emergents etc but in general do it when the plant is actively growing. I have had to avoid several kinds of pasture chemicals to avoid killing arrowleaf clover.
To increase your chances of a good kill I suggest you positively identify the clover as to species (lots of clovers) and get a recommendation from your extension agent or reputable chemical manufacturer, like Dow.
Although label info may be, in general, correct it is often a chore to try to find what you need without consulting a ggod source such as the extension service or a specialty chemical mfg who makes herbicides.
If you are into NATURAL then...
A multi-year program of applying natural corn gluten meal, which will have the added benefit of killing all of your other seed-based weeds and adding nitrogen to the soil, too. Remember that corn gluten meal will suppress germination of grass seeds as well, so time your applications away from reseedings.
Pat