Function of church, hiding old wickedness? (6)
I believe the answer is:
cosine
'function' is the definition.
(cosine is a kind of function)
'church hiding old wickedness?' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'hiding' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wickedness?' becomes 'sin' ('sin' can be a synonym of 'wickedness').
'o'+'sin'='osin'
'ce' going around 'osin' is 'COSINE'.
'of' is the link.
(Other definitions for cosine that I've seen before include "Trig function" , "Trigonometric function" , "Ratio in trigonometry" , "Trigonometrical ratio" , "Trig ratio" .)