Stop weirdo letters (4)
I believe the answer is:
post
'letters' is the definition.
(letters in the mail or post)
'stop weirdo' is the wordplay.
'weirdo' is an anagram indicator (an unusual or weird ordering of the letters).
'stop' is an anagram of 'POST'.
(Other definitions for post that I've seen before include "Leave message online" , "Appointment (with the Royal Mail?)" , "A column" , "Prevent (cheque) being cashed immediately" , "trading establishment" .)