Finely grind fennel seeds in an electric coffee/spice grinder or with a mortar and pestle, then stir together with cider and sugar until sugar is dissolved. Pour mixture through a fine sieve into an 8- to 9-inch baking pan, discarding solids.
Freeze, stirring and crushing lumps with a fork every 30 minutes, until evenly frozen, about 2 hours total. Scrape with a fork to lighten texture, crushing any lumps.
Serve immediately or freeze, covered, up to 3 days (rescrape to lighten texture again if necessary).