Ethnic oil works from the Stone Age (9)
I believe the answer is:
neolithic
'from the stone age' is the definition.
(I've seen this before)
'ethnic oil works' is the wordplay.
'works' indicates anagramming the letters (work can mean to twist or convulse).
'ethnic'+'oil'='ethnicoil'
'ethnicoil' is an anagram of 'NEOLITHIC'.
(Other definitions for neolithic that I've seen before include "Relating to the later Stone Age" , "about when stone weapons prevailed" , "Description of old age" , "Description of ancient people" , "from Skara Brae?" .)