A European supplier of oil needing treatment (6)
I believe the answer is:
unwell
'needing treatment' is the definition.
'a european supplier of oil' is the wordplay.
'a european' becomes 'un' ('a' translated into French).
'supplier of oil' becomes 'well' (I can't explain this - if you can you should believe this answer much more).
'un'+'well'='UNWELL'
Can you help me to learn more?
(Other definitions for unwell that I've seen before include "Ill, not in good health" , "Somewhat ill" , "Sick, not in good health" , "funny" , "Indisposed" .)