Flower’s unusual drainage (8)
I believe the answer is:
gardenia
'flower's' is the definition.
(I have seen 'Fragrant flower ' mean 'gardenia' so perhaps 'flower' could also mean 'gardenia')
'unusual drainage' is the wordplay.
'unusual' is an anagram indicator.
'drainage' with letters rearranged gives 'GARDENIA'.
(Other definitions for gardenia that I've seen before include "Bad drainage produces a fragrant flower" , "Fragrant tree or shrub" , "Drainage (anag) -- plant" , "bloomer!" , "Waxlike flower" .)