Carnival in France is held by motor-racing authority (6)
I believe the answer is:
fiesta
'carnival' is the definition.
(thesaurus)
'in france is held by motor-racing authority' is the wordplay.
'in france is' becomes 'est' ('is' in French).
'held by' is an insertion indicator.
'motor-racing authority' becomes 'fia' (Federation Internationale de l'Automobile).
'est' placed inside 'fia' is 'FIESTA'.
(Other definitions for fiesta that I've seen before include "Festival, fete" , "celebration abroad" , "Latin American celebration" , "Holiday; carnival" , "Continental knees-up" .)