Periodic function (4)
I believe the answer is:
sine
'periodic 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 "Without; maths function" , "Trigonometrical function" , "Indefinitely, with no appointed date - . . . . die" , "Latin tag for indefinitely - . . . . die" , "Indispensable condition" .)