The French sex joke reversed wedding (7)
I believe the answer is:
nuptial
'wedding' is the definition.
(I've seen this before)
'the french sex joke reversed' is the wordplay.
'the french' becomes 'la' (one of the French words for 'the').
'sex' becomes 'it' (a common euphemism).
'joke' becomes 'pun' (pun is a type of joke).
'reversed' says the letters should be written in reverse.
'la'+'it'+'pun'='laitpun'
'laitpun' back-to-front is 'NUPTIAL'.
(Other definitions for nuptial that I've seen before include "To do with marriage or a wedding ceremony" , "Marital" , "Relating to matrimony" , "Matrimonial" , "Relating to weddings" .)