Imitation stone that's a national emblem (8)
I believe the answer is:
shamrock
'a national emblem' is the definition.
(I've seen this before)
'imitation stone' is the wordplay.
'imitation' becomes 'sham' (sham is a kind of imitation).
'stone' becomes 'rock' (synonyms).
'sham'+'rock'='SHAMROCK'
'that's' is the link.
(Other definitions for shamrock that I've seen before include "symbolic of Ireland" , "Mash cork into the emblem" , "Clover-like plant" , "Irish national symbol" , "Sign of first part of 10 [IRISH [SEA]]" .)