Start of European flower festival (6)
I believe the answer is:
easter
'festival' is the definition.
(I've seen this before)
'start of european flower' is the wordplay.
'start of' suggests taking the first letters.
'flower' becomes 'aster' (aster is a kind of flower**).
The first letter of 'european' is 'e'.
'e'+'aster'='EASTER'
(Other definitions for easter that I've seen before include "Egg-rolling time" , "Christian festival in Spring" , "festival time" , "- Day; - bunny" , "Sort of tide" .)