Source of crude painting, in all likelihood (3,4)
I believe the answer is:
oil well
'source of crude' is the definition.
(I've seen this before)
'painting in all likelihood' is the wordplay.
'painting' becomes 'oil' (oil painting is a kind of painting).
'in all likelihood' becomes 'well' (I am not sure about this - if you are sure you should believe this answer much more).
'oil'+'well'='OIL-WELL'
Can you help me to learn more?
(Other definitions for oil well that I've seen before include "Boring made for extracting petroleum" , "as result of a strike" , "How to minimise friction" , "crude container" , "Shaft beneath a nodding donkey?" .)