Reciprocal of sine (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
cosec
(Other definitions for cosec that I've seen before include "company secretary" , "(In mathematics) the reciprocal of sine (abbr)" , "Inverse of sine (abbr.)" , "(In maths) reciprocal of sine (abbr)" , "Trig function" .)