Clue: English mathematician's trick with a variable (6)
Answer: CONWAY
Explanation: Definition: English mathematician
CON (trick) + W[ith] A Y – John Conway, formerly at Cambridge, and latterly at Princeton, who is well known for his contributions to many areas of mathematics, including the "recreational side"; he's perhaps best known for his Game of Life. I used to know him quite well when I was a graduate student at Cambridge.