From an ancient area, as oil may be (5)
I believe the answer is:
incan
'oil may be' is the definition.
The definition suggests a singular noun which matches the answer.
'from an ancient area' is the wordplay.
'from' becomes 'in' (I've seen this in other clues).
'an ancient area' becomes 'can' (can is a kind of area. I am not sure about the 'ancient' bit.).
'in'+'can'='INCAN'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for incan that I've seen before include "Imperialist of the past" , "doing time?" , "like former S American civilisation" , "Of former Peruvian race" , "Relating to a pre-conquest people of Peru" .)