Grecian map adapted by one of The Crusaders (10)
I believe the answer is:
campaigner
'one of the crusaders' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'grecian map adapted' is the wordplay.
'adapted' is an anagram indicator.
'grecian'+'map'='grecianmap'
'grecianmap' is an anagram of 'CAMPAIGNER'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for campaigner that I've seen before include "Active supporter of a movement" , "Activist" , "Advocate" , "Person fighting" , "Militant" .)