Plant border in great pain (8)
I believe the answer is:
agrimony
'plant' is the definition.
(agrimony is a kind of plant)
'border in great pain' is the wordplay.
'border' becomes 'rim' (I've seen this before).
'in' is an insertion indicator.
'great pain' becomes 'agony' (I've seen this before).
'rim' placed within 'agony' is 'AGRIMONY'.
(Other definitions for agrimony that I've seen before include "Plant with yellow flower-spikes" , "Growth" .)