Plant displayed by a large green stone (7)
I believe the answer is:
alecost
'plant' is the definition.
(alecost is a kind of plant)
'a large green stone' is the wordplay.
'large' becomes 'l'.
'green' becomes 'eco' (abbreviation for ecological).
'stone' becomes 'st'.
'a'+'l'+'eco'+'st'='ALECOST'
'displayed by' is the link.
(Other definitions for alecost that I've seen before include "Herb" , "plant" , "used to provide flavouring for beer" .)