Comprehensive sees promotion for the lowliest of blossoms (6)
I believe the answer is:
floral
'of blossoms' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for floral that I've seen before include "Decorated with blooms" , "Sort of tribute or dance" , "Decorated with flowers" , "Of flowers, like wreath" , "Place for seaside holiday" .)