Lops head off plant (8)
I believe the answer is:
asphodel
'plant' is the definition.
(type of flower)
'lops head off' is the wordplay.
'off' indicates an anagram.
'lops'+'head'='lopshead'
'lopshead' anagrammed gives 'ASPHODEL'.
(Other definitions for asphodel that I've seen before include "Shop deal (anag.)" , "Old shape (anag) - plant" , "blooms from Elysian fields" , "Mediterranean plant" , "Flower of lily family - old heaps (anag)" .)