The term 'TONIC MIXER' is a combination of two words, 'TONIC' and 'MIXER'. TONIC is a type of beverage that is usually made with carbonated water, sugar, and quinine, and is often used as a MIXER for alcoholic drinks. A MIXER is an ingredient added to a drink to enhance its flavor or to make it more palatable. In this case, the solution for the crossword is GIN, which is an alcoholic spirit made from juniper berries. GIN is often used as a MIXER in TONIC drinks, making it the perfect answer for this crossword puzzle.