Protest monster originally received in post (13)
I believe the answer is:
demonstration
'protest' is the definition.
(I know that protest can be written as demonstration)
'monster originally received in post' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'monster' is 'emonstr' which is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for demonstration that I've seen before include "Protest march/meeting" , "It's to voice protest" , "March; proof" , "Protest march" , "Protest meeting" .)