Piece of a dress, a strand of hair and poison? (7)
I believe the answer is:
hemlock
'poison?' is the definition.
(I know that hemlock is a type of phytotoxin)
'piece of a dress a strand of hair' is the wordplay.
'piece of a dress' becomes 'hem' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a strand of hair' becomes 'lock' (I've seen this before).
'hem'+'lock'='HEMLOCK'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for hemlock that I've seen before include "Highly poisonous relative of parsley" , "Socrates's poison" , "Poisonous plant" , "Poisonous weed" , "preparation to kill" .)