base, bilge, heel, sole, underbelly.
undersize, undertide.
actinide, allied, amide, apartheid, arsenide, backside, backslide, beatified, bedside, blindside, broadside, bromide, burnside, carbide, certified, citywide, classified, cockeyed, codified, countryside.
Your query has returned 390 words, which include anagrams of undersides as well as other shorter words that can be made using the letters included in undersides.
The words in this list can be used in games such as Scrabble, Words with Friends, Wordscapes and other similar games.