Initially English flower (an annual) (6)
I believe the answer is:
easter
'an annual' is the definition.
'initially english flower' is the wordplay.
'initially' indicates taking the first letters.
'flower' becomes 'aster' (type of flower).
The first letter of 'english' is 'e'.
'e'+'aster'='EASTER'
(Other definitions for easter that I've seen before include "Resurrection festival" , "Christian celebration for Teresa" , "Christian celebration at the end of Lent" , "Teaser about Christian festival" , "Time to parade" .)