Fake stone plant (8)
I believe the answer is:
shamrock
'plant' is the definition.
(shamrock is a kind of plant)
'fake stone' is the wordplay.
'fake' becomes 'sham' (synonyms).
'stone' becomes 'rock' (synonyms).
'sham'+'rock'='SHAMROCK'
(Other definitions for shamrock that I've seen before include "Ireland's special trefoil" , "Clover leaf" , "Clover-like plant" , "Irish national symbol" , "Irish national emblem" .)