Sea boy tours new territory (8)
I believe the answer is:
mainland
'territory' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'sea boy tours new' is the wordplay.
'sea' becomes 'main' (synonyms).
'boy' becomes 'lad' (informal term for boy).
'tours' indicates putting letters inside (to tour can mean to go around).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'lad' going around 'n' is 'land'.
'main'+'land'='MAINLAND'
Can you help me to learn more?
(Other definitions for mainland that I've seen before include "Area excluding offshore islands" , "A country's principal as opposed to its islands" , "Principle country, not an island" , "Country or continent, as distinguished from an island" , "Part of territory excluding offshore islands" .)