Note about sex and religion (5)
I believe the answer is:
faith
'religion' is the definition.
(I've seen this before)
'note about sex' is the wordplay.
'note' becomes 'fah' (fourth note in sol-fa notation, anglicized version**).
'about' is an insertion indicator.
'sex' becomes 'it' (a common euphemism).
'fah' placed around 'it' is 'FAITH'.
'and' acts as a link.
(Other definitions for faith that I've seen before include "Total trust" , "Strong belief in a supernatural power" , "A woman" , "Religious belief" , "Trust in someone" .)