The term 'duvet filler' refers to the material used to fill a duvet, which is a type of bedding that is filled with down or synthetic fibers and covered with a removable cover. The crossword solution for this term is 'EIDER, ' which is a species of duck that produces a soft, downy feather used as a filling for duvets. Eiderdown is highly valued for its insulating properties and is considered one of the finest natural fillers for duvets.