Stop sending letters (4)
I believe the answer is:
post
'letters' is the definition.
(letters in the mail or post)
'stop sending' is the wordplay.
'sending' indicates an anagram (letters are sent to different positions).
'stop' anagrammed gives 'POST'.
(Other definitions for post that I've seen before include "Prevent (cheque) being cashed immediately" , "a placement: it's > part of one's goal" , "type of support" , "delivery" , "Member < to comment on website" .)