Plant fastening mechanism underneath border (7)
I believe the answer is:
hemlock
'plant' is the definition.
(hemlock is a kind of plant)
'fastening mechanism underneath border' is the wordplay.
'fastening' becomes 'lock' (lock is a kind of fastening).
'mechanism underneath' is a charade indicator (letters next to each other) (I've seen this in other clues).
'border' becomes 'hem' (I've seen this before).
'lock' put after 'hem' is 'HEMLOCK'.
(Other definitions for hemlock that I've seen before include "preparation to kill" , "Poisonous drug" , "Socratic poison" , "Poisonous weed" , "Socrates's poison" .)