Reciprocal of cosine (6)
I believe the answer is:
secant
'reciprocal of cosine' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for secant that I've seen before include "Line that intercepts a curve" , "Ratio of the hypotenuse to the adjacent side in a right-angled triangle" , "Line that intersects a curve at two or more points" , "triangular relationship" , "Trig function" .)