Shrub needing fantastic drainage (8)
I believe the answer is:
gardenia
'shrub' is the definition.
(I know that Gardenia is a type of shrub)
'fantastic drainage' is the wordplay.
'fantastic' is an anagram indicator.
'drainage' anagrammed gives 'GARDENIA'.
'needing' acts as a link.
(Other definitions for gardenia that I've seen before include "Drainage for shrub with white or yellow flowers" , "Shrub such as the Cape jasmine" , "Bad drainage produces a fragrant flower" , "Shrub with fragrant flowers - in Grade A (anag)" , "Drainage (anag) -- plant" .)