Stop sending mail (4)
I believe the answer is:
post
'mail' is the definition.
(I know that mail can be written as post)
'stop sending' is the wordplay.
'sending' indicates anagramming the letters (letters are sent to different positions).
'stop' anagrammed gives 'POST'.
(Other definitions for post that I've seen before include "may be winning" , "A column" , "Member < to comment on website" , "Position; mail" , "ob" .)