Stop tearing letters! (4)
I believe the answer is:
post
'letters' is the definition.
(letters in the mail or post)
'stop tearing' is the wordplay.
'tearing' indicates an anagram.
'stop' with letters rearranged gives 'POST'.
(Other definitions for post that I've seen before include "Publicly display; send" , "Station" , "a placement: it's > part of one's goal" , "Leave message online" , "may be winning" .)