Noel's parasitic growth transformed motel site (9)
I believe the answer is:
mistletoe
'noel's parasitic growth' is the definition.
(plant associated with Christmas or Noel)
'transformed motel site' is the wordplay.
'transformed' indicates anagramming the letters.
'motel'+'site'='motelsite'
'motelsite' with letters rearranged gives 'MISTLETOE'.
(Other definitions for mistletoe that I've seen before include "People kiss under this plant" , "Tree parasite" , "Christmas berry" , "Valley of the -, best seller by Jacqueline Susann" , "tempting a smack?" .)