Cease sending post (4)
I believe the answer is:
stop
'cease' is the definition.
(synonyms)
'sending post' is the wordplay.
'sending' indicates anagramming the letters (letters are sent to different positions).
'post' anagrammed gives 'STOP'.
(Other definitions for stop that I've seen before include "Plug; cease" , "Come to a halt" , "Organ knob; get bus here" , "Blockage" , "Cease; organ voice" .)