Four seen in sensual fiesta (8)
I believe the answer is:
carnival
'fiesta' is the definition.
(both can mean a celebration)
'four seen in sensual' is the wordplay.
'four' becomes 'IV' (Roman numerals for four).
'seen in' indicates putting letters inside.
'sensual' becomes 'carnal' (synonyms).
'iv' put into 'carnal' is 'CARNIVAL'.
(Other definitions for carnival that I've seen before include "Festive period" , "Notting Hill event" , "Pre-Lent jollity" , "Fiesta" , "Mardi Gras, for example" .)