Large numbers leaving northern border in chaos (6)
I believe the answer is:
mayhem
'chaos' is the definition.
(I've seen this before)
'large numbers leaving northern border' is the wordplay.
'large numbers' becomes 'many' (many means a large number).
'leaving' suggests deleting specific letters.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'border' becomes 'hem' (I've seen this before).
'many' with 'n' taken away is 'may'.
'may'+'hem'='MAYHEM'
'in' is the link.
(Other definitions for mayhem that I've seen before include "violence" , "Chaos, violent disorder" , "Total chaos" , "Havoc - chaos" , "wilful infliction of injury" .)