Fiesta motor, new, a line accommodating four (8)
I believe the answer is:
carnival
'fiesta' is the definition.
(carnival is a kind of fiesta)
'motor new a line accommodating four' is the wordplay.
'motor' becomes 'car' (I've seen this before).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'line' becomes 'l'.
'accommodating' indicates putting letters inside.
'four' becomes 'IV' (roman numeral).
'n'+'a'+'l'='nal'
'nal' placed around 'iv' is 'nival'.
'car'+'nival'='CARNIVAL'
(Other definitions for carnival that I've seen before include "Festive entertainment" , "Festive fair" , "Period of public revelry" , "Notting Hill event" , "Dvorak overture" .)