Frost damage should not be lethal to alfalfa.  Alfalfa, if established at all, has a lot of growing points, and even if the primary bud is damaged, the secondary buds will take over and resume growth. There is some delay in growth expected as the plants recover and first cutting yield may be a little lower. Clipping (like you would do for heavy weevil damage) is not recommended. Keep the alfalfa healthy from a fertility point of view and recovery should be good!