Security needed for place on the border (6)
I believe the answer is:
pledge
'security' is the definition.
(something given as security when taking out a loan)
'place on the border' is the wordplay.
'place' becomes 'pl' (abbreviation used in postal addresses).
'on' is a charade indicator (letters next to each other).
'the border' becomes 'edge' (synonyms).
'pl'+'edge'='PLEDGE'
'needed for' acts as a link.
(Other definitions for pledge that I've seen before include "Promise, word of honour" , "assurance" , "Vow; security" , "Give one's word" , "Vow, affirm" .)