Sex before church is a sin (4)
I believe the answer is:
vice
'sin' is the definition.
(thesaurus)
'sex before church' is the wordplay.
'sex' becomes 'VI' (VI in Roman numerals - 'sex' is 'six' in Latin).
'before' means one lot of letters go next to another.
'church' becomes 'CE' (Church of England).
'vi'+'ce'='VICE'
'is a' is the link.
(Other definitions for vice that I've seen before include "Fault - gripping tool" , "Gripper - in place of - immorality" , "Wicked habit" , "Gripper - sin" , "Tool with gripping jaws" .)