Plant imitation jewel (8)

I believe the answer is:

shamrock

'plant' is the definition.
(shamrock is a kind of plant)

'imitation jewel' is the wordplay.
'imitation' becomes 'sham' (sham is a kind of imitation).
'jewel' becomes 'rock' (informal term for a precious stone).
'sham'+'rock'='SHAMROCK'

(Other definitions for shamrock that I've seen before include "Mash cork into the emblem" , "Irish national symbol" , "Sign of first part of 10 [IRISH [SEA]]" , "Clover leaf" , "Clover-like plant" .)

I've seen this clue in The Telegraph.

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