Girl at the border causing havoc (6)
I believe the answer is:
mayhem
'havoc' is the definition.
(I know that havoc can be written as mayhem)
'girl at the border' is the wordplay.
'girl' becomes 'may' ().
'at' means one lot of letters go next to another.
'the border' becomes 'hem' (I've seen this before).
'may'+'hem'='MAYHEM'
'causing' acts as a link.
(Other definitions for mayhem that I've seen before include "Havoc - chaos" , "Scene of great disorder" , "Pandemonium" , "State of great confusion and disorder" , "Chaos, violent disorder" .)