Shrub needing drainage badly (8)
I believe the answer is:
gardenia
'shrub' is the definition.
(gardenia is a kind of shrub)
'drainage badly' is the wordplay.
'badly' is an anagram indicator.
'drainage' is an anagram of 'GARDENIA'.
'needing' acts as a link.
(Other definitions for gardenia that I've seen before include "Shrub with fragrant flowers - in Grade A (anag)" , "Bad drainage can produce a fragrant flower" , "Shrub; drainage (anag.)" , "Fragrant tree or shrub" , "Drainage (anag) -- plant" .)