Parasitic plant that's disastrous to elm site (9)
I believe the answer is:
mistletoe
'parasitic plant' is the definition.
(mistletoe is a kind of parasitic plant)
'disastrous to elm site' is the wordplay.
'disastrous' indicates anagramming the letters.
'to'+'elm'+'site'='toelmsite'
'toelmsite' with letters rearranged gives 'MISTLETOE'.
'that's' acts as a link.
(Other definitions for mistletoe that I've seen before include "White-berried parasite" , "Kissed-under plant" , "Stole mite from the Christmas plant" , "Parasitic shrub for Christmas" , "Parasitic plant" .)