Cosecant's reciprocal (4)
I believe the answer is:
sine
'cosecant's reciprocal' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sine that I've seen before include "Latin tag for indefinitely - . . . . die" , "Indefinitely, with no appointed date - . . . . die" , "Indispensable condition" , "Trigonometric function" , "Trig function" .)