Certainly it's not far eastern in origin (7)
I believe the answer is:
nearest
'certainly' is the definition.
Both the answer and definition are adverbs. Perhaps they are linked in a way I don't understand?
'not far eastern in origin' is the wordplay.
'eastern' anagrams to 'NEAREST'.
However, I'm unsure how this is indicated.
'it's' is the link.
Can you help me to learn more?
(Other definitions for nearest that I've seen before include "most like Scrooge at first" , "Next" , "Most closely related" , "Most adjacent" , "With dearest, they're our loved ones" .)