Jack Brown is popular in Scotland (6)
I believe the answer is:
tartan
'popular in scotland' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'jack brown' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'brown' becomes 'tan' (tan is a kind of brown).
'tar'+'tan'='TARTAN'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for tartan that I've seen before include "Woollen checked material" , "Scots plaid" , "Woollen fabric with plaid design" , "Clan fabric" , "Scots cloth pattern" .)