Authentic roasted grain oil (8)
I believe the answer is:
'authentic' is the definition.
'roasted grain oil' is the wordplay.
'grain oil' can be anagrammed to 'ORIGINAL'.
However, I am not sure how this is indicated.
Can you help me to learn more?
(Other definitions for original that I've seen before include "Creative, new in design" , "Not derived or copied form something else" , "Phototype" , "The first, not a copy" , "Not an imitation, the first" .)