Hardy siliceous outcrop verges on plant (6)
I believe the answer is:
hyssop
'plant' is the definition.
(hyssop is a kind of plant)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for hyssop that I've seen before include "Herb in the mint family" , "little blue flowers" , "Aromatic plant" , "Small bushy plant with bitter minty leaves" , "Plant with minty leaves used in cooking or as medicine" .)