Post Office is working to shut up malicious writer (6,3)
I believe the answer is:
poison pen
'post office is working to shut up malicious writer' is the definition.
I can't tell whether this defines the answer.
'post office is working to shut up' is the wordplay.
'post office' becomes 'po' (abbreviation).
'working' becomes 'on' (e.g. a machine is working or on).
'to shut up' becomes 'pen' (pen can mean to shut in or enclose).
'po'+'is'+'on'+'pen'='POISON-PEN'
Can you help me to learn more?
(Other definitions for poison pen that I've seen before include "unpleasant sort of letter" , "Writer of malicious anonymous letters" .)