Horticultural category (6)
I believe the answer is:
annual
'horticultural category' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for annual that I've seen before include "Occuring once a year" , "Happening once each year" , "Book published yearly" , "Plant that lives and dies within one year" , "Every twelve months" .)