Rich source of advice to lubricate thoroughly (3,4)
I believe the answer is:
oil well
'rich source' is the definition.
'oil well' can be an answer for 'source' (I have seen 'Source of fuel' mean 'oil well' so perhaps 'source' could also mean 'oil well'). I am unsure of the 'rich' bit.
'advice to lubricate thoroughly' is the wordplay.
I cannot really understand how this works, but
'lubricate' could be 'oil' (synonyms) and 'oil' is located in the answer.
'thoroughly' could be 'well' (I've seen this in another clue) and 'well' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'of' acts as a link.
Can you help me to learn more?
(Other definitions for oil well that I've seen before include "Gusher, perhaps" , "Boring made for extracting petroleum" , "Shaft to extract fuel" , "source of serious income" , "Whence petroleum is extracted" .)