Function in which one's inducted into religious group (6)
I believe the answer is:
secant
'function' is the definition.
(I know that secant is a type of circular function)
'one's inducted into religious group' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'inducted into' means one lot of letters goes inside another.
'religious group' becomes 'sect' (eg a religious sect).
'an' put within 'sect' is 'SECANT'.
'in which' acts as a link.
(Other definitions for secant that I've seen before include "Line that intersects a curve at two or more points" , "Cutting line (geometry)" , "Trig function" , "Line that intercepts a curve" , "triangular relationship" .)