Post also including north country’s bulk (8)
I believe the answer is:
mainland
'country's bulk' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'post also including north' is the wordplay.
'post' becomes 'mail' (synonyms).
'also' becomes 'and' (both can mean in addition).
'including' is an insertion indicator.
'north' becomes 'N' (abbreviation).
'mail'+'and'='mailand'
'mailand' placed around 'n' is 'MAINLAND'.
Can you help me to learn more?
(Other definitions for mainland that I've seen before include "Principle country, not an island" , "Part of territory excluding offshore islands" , "Principal part of a country, not an island" , "Country or continent, as distinguished from an island" , "Area excluding offshore islands" .)