Reportedly purchase some African lilies primarily for celebration (8)
I believe the answer is:
biennial
'celebration' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the remainder of the clue.
(Other definitions for biennial that I've seen before include "Happening every second year" , "Plant with a two-year life cycle" , "Taking place every other year" , "Occurring every second year" , "Coming every other year" .)