Silver edge on unknown herb (8)
I believe the answer is:
agrimony
'herb' is the definition.
(I know that agrimony is a type of herb)
'silver edge on unknown' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'edge' becomes 'rim' (rim is a kind of edge).
'unknown' becomes 'y' (algebra).
'ag'+'rim'+'on'+'y'='AGRIMONY'
(Other definitions for agrimony that I've seen before include "Growth" , "Plant with yellow flower-spikes" .)