Border security that's deadly (7)

I believe the answer is:

hemlock

I'm a little stuck... Click here to teach me more about this clue!

'that's deadly' is the definition.
I can't tell whether this definition defines the answer.

'border security' is the wordplay.
'border' becomes 'hem' (I've seen this before).
'security' becomes 'lock'.
'hem'+'lock'='HEMLOCK'

Can you help me to learn more?

(Other definitions for hemlock that I've seen before include "Poisonous plant" , "Poisonous weed" , "Socratic poison" , "preparation to kill" , "Highly poisonous relative of parsley" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app