The crossword solution for the term 'swirl in a brook' is EDDY, which is a four-letter word. An EDDY is a swirling body of water that forms in a stream or brook due to the presence of an obstacle or change in the current. Eddies can form in both slow and fast-moving water, and they often contain debris and sediment. Eddies are important for aquatic life, as they provide shelter and food for fish and other aquatic organisms.