A growing source of border security? (7)
I believe the answer is:
hemlock
'a growing source' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'border security?' is the wordplay.
'border' becomes 'hem' (I've seen this before).
'security?' becomes 'lock'.
'hem'+'lock'='HEMLOCK'
'of' is the link.
Can you help me to learn more?
(Other definitions for hemlock that I've seen before include "Poisonous drug" , "Poisonous plant" , "Socrates's poison" , "Poison killing Socrates" , "Poisonous weed" .)