Celebration if taken over by French is awesome at the outset (6)
I believe the answer is:
fiesta
'celebration' is the definition.
(fiesta is a kind of celebration)
'if taken over by french is awesome at the outset' is the wordplay.
'taken over' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'french is' becomes 'est' ('is' in the French language).
'at the outset' indicates taking the first letters.
The first letter of 'awesome' is 'a'.
'if' back-to-front is 'fi'.
'fi'+'est'+'a'='FIESTA'
(Other definitions for fiesta that I've seen before include "Public whoopee in Spain" , "(Spanish) religious festival" , "Religious festival in Spain" , "celebration abroad" , "Spanish party" .)