Scotch Elm I test with old parasitic growth (9)
I believe the answer is:
mistletoe
'parasitic growth' is the definition.
'scotch elm i test with old' is the wordplay.
'scotch' indicates an anagram (I've seen this in other clues).
'with' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'elm'+'i'+'test'='elmitest'
'elmitest' with letters rearranged gives 'mistlete'.
'mistlete' enclosing 'o' is 'MISTLETOE'.
(Other definitions for mistletoe that I've seen before include "it may get you a few smackers" , "Plant with white berries, facilitates kissing" , "tempting a smack?" , "Evergreen plant" , "Kissed-under plant" .)