Clue: Oxford mathematician is evasive about subtracting transcendental number (7)
Answer: DODGSON
Explanation: Definition: Oxford mathematician
 "dodges"(is evasive/avoids) + ON(about/with reference to) **minus**(subtracting) "e"(a transcendental number, that which cannot be understood by ordinary reasoning, in this case the number that is used as the base of natural logarithms in mathematics).
Defn: …, whose pen name was Lewis Carroll.