Rich source of oil also including diamonds (4-2-2)
I believe the answer is:
well-to-do
'rich' is the definition.
(I've seen this before)
'source of oil also including diamonds' is the wordplay.
'source of oil' becomes 'well' (I am not sure about this - if you are sure you should believe this answer much more).
'also' becomes 'too' (synonyms).
'including' is an insertion indicator.
'diamonds' becomes 'd' (abbreviation in bridge say).
'too' placed around 'd' is 'todo'.
'well'+'todo'='WELL-TO-DO'
Can you help me to learn more?
(Other definitions for well-to-do that I've seen before include "Moderately wealthy" , "Fairly rich, prosperous" , "Financially comfortable" , "with a lot of funds" , "Rich; old towel (anag.)" .)