Annual flower for a sweetheart! (6)
I believe the answer is:
easter
'annual' is the definition.
'flower for a sweetheart' is the wordplay.
'flower' becomes 'aster' (type of flower).
'for' means one lot of letters go next to another (I've seen this in other clues).
'a sweetheart' becomes 'e' (the heart/centre of 'sweet').
'aster' put after 'e' is 'EASTER'.
(Other definitions for easter that I've seen before include "Teaser (anag.)" , "Christian feast of the Resurrection" , "Resurrection festival" , "Sort of tide" , "Annual religious festival" .)