Post a second notice (6)
I believe the answer is:
assign
'post' is the definition.
(I know that post is a more specific form of the action assign)
'a second notice' is the wordplay.
'second' becomes 's' (common abbreviation).
'notice' becomes 'sign' (I've seen this before).
'a'+'s'+'sign'='ASSIGN'
(Other definitions for assign that I've seen before include "Select and appoint for task" , "Appoint to a duty" , "Allot, fix, appoint" , "Entrust or convey to" , "Nominate" .)