Instruction to the mechanic might be a bore (3,4)
I believe the answer is:
oil well
'a bore' is the definition.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for oil well that I've seen before include "Bore made for fuel" , "Instruction on machine perhaps" , "source of wealth" , "Borehole for petroleum" , "Gusher, perhaps" .)