Letters to be sent to remote settlement (7)
I believe the answer is:
outpost
'remote settlement' is the definition.
(I've seen this in another clue)
'letters to be sent' is the wordplay.
'letters' becomes 'post' (letters in the mail or post).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'be sent' becomes 'out' (I can't justify this - if you can you should believe this answer much more).
'post' put after 'out' is 'OUTPOST'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for outpost that I've seen before include "Small camp some distance away from the main army" , "Outlying settlement" , "Remote military camp" , "distant location" , "Small group of soldiers" .)