Fourier series function (4)
I believe the answer is:
sine
'fourier series function' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sine that I've seen before include "Trigonometrical function" , "Trigonometric function" , "Indispensable condition" , "Latin tag for indefinitely - . . . . die" , "Without; maths function" .)