Stop sorting the mail (4)
I believe the answer is:
post
'mail' is the definition.
(synonyms)
'stop sorting' is the wordplay.
'sorting' indicates anagramming the letters.
'stop' is an anagram of 'POST'.
'the' is the link.
(Other definitions for post that I've seen before include "a placement: it's > part of one's goal" , "Prevent (cheque) being cashed immediately" , "trading establishment" , "Send; job" , "picket" .)