The term 'Grand Slam Foursome' refers to a group of four people who have each won all four major golf tournaments, known as the Grand Slam. This includes the Masters, the U.S. Open, the British Open, and the PGA Championship. The crossword solution for this term is 'RUNS, ' which can refer to the act of running or the act of playing golf. In this context, it is likely referring to the latter, as the foursome has achieved a grand slam in golf.