Science of numbers (11)
'science of numbers' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mathematics that I've seen before include "a school subject" , "The science of numbers etc" , "Sums" , "In which pupils may have to add" , "Numerical science" .)
(Other definitions for maths that I've seen before include "Algebra, geometry, calculus etc (abbr)" , "Adding and subtraction, etc" , "It concerns numbers, in short" , "The science of number, quantity and space" , "Arithmetic etc" .)