Tenth article about sex (5)
I believe the answer is:
tithe
'tenth' is the definition.
(a tenth of one's income given to a church)
'article about sex' is the wordplay.
'article' becomes 'the' (the definite article in English).
'about' means one lot of letters goes inside another.
'sex' becomes 'it' (a common euphemism).
'the' placed around 'it' is 'TITHE'.
(Other definitions for tithe that I've seen before include "Former church tax" , "One tenth - old church tax" , "Tenth (of one's earnings)" , "that was given to clergyman" , "10% tax" .)