Released mail for border settlement (7)
I believe the answer is:
outpost
'border settlement' is the definition.
'outpost' can be an answer for 'settlement' (outpost is a kind of settlement). I am not sure about the 'border' bit.
'released mail' is the wordplay.
'released' becomes 'out' (eg out of prison).
'mail' becomes 'post' (synonyms).
'out'+'post'='OUTPOST'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for outpost that I've seen before include "Distant military camp" , "Remote military camp" , "isolated settlement" , "Small camp some distance away from the main army" , "Remote part of the country" .)