Plant fake diamond (8)
I believe the answer is:
'plant' is the definition.
(shamrock is a kind of plant)
'fake diamond' is the wordplay.
'fake' becomes 'sham' (sham can mean fake or pretend).
'diamond' becomes 'rock' (I can't justify this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for shamrock that I've seen before include "The national Irish trefoil" , "Irish national symbol" , "Sign of first part of 10 [IRISH [SEA]]" , "Native Irish trefoil" , "Clover native to Ireland" .)