The term 'duck down' can be broken down into two crossword solutions: EIDER (5 letters) and HIDE (4 letters). EIDER is a type of duck that is native to the Northern Hemisphere, and its feathers are used to make down insulation for clothing. HIDE is a verb meaning to conceal or cover up something. Therefore, the phrase 'duck down' can be interpreted as a combination of these two words, suggesting the act of concealing oneself with the feathers of an EIDER duck.