Sex returns in erotic festival (8)
I believe the answer is:
carnival
'festival' is the definition.
(I know that carnival is a type of festival)
'sex returns in erotic' is the wordplay.
'sex' becomes 'VI' (VI in Roman numerals - 'sex' is 'six' in Latin).
'returns' is a reversal indicator.
'in' is an insertion indicator.
'erotic' becomes 'carnal' (I've seen this before).
'vi' reversed gives 'iv'.
'iv' put within 'carnal' is 'CARNIVAL'.
(Other definitions for carnival that I've seen before include "Period of public revelry" , "Festive fair" , "Pre-Lent jollity" , "Notting Hill event" , "Meddle" .)