A Dark Clump is a material found in Tears of the Kingdom.
Tears of the Kingdom
"An unusual substance, it's cold to the touch, and it seems as though it's filled with pitch-black darkness. Use it in cooking to get a gloom-resistant effect."
Dark Clumps can be obtained for 10 Poes each at Bargainer Statues, who can all sell Link a maximum of 10, and can be sold to other shop-vendors in Hyrule for 5 Rupees. They are also dropped upon defeating Gloom Spawn and/or Phantom Ganons.
Link can use Dark Clumps in cooking for a Gloom Resistance effect, although cooking them alone results in Dubious Food. Recipes which include Dark Clumps include Dark Stew, Dark Soup, Dark Curry, Dark Rice Ball and Dark Cake; as well as the Warding variants thereof. A maximum of four Dark Clumps can be used in a meal, providing the maximum Gloom Resistance of 3 hearts.