Painting efficiently's a boring source of income? (3,4)
I believe the answer is:
oil well
'a boring source of income?' is the definition.
(an oil well bores into the ground)
'painting efficiently's' is the wordplay.
'painting' becomes 'oil' (oil painting is a kind of painting).
'efficiently' becomes 'well'.
'oil'+'well'='OIL-WELL'
(Other definitions for oil well that I've seen before include "Sunken source of fuel" , "Maintenance advice for metal hinges" , "How to avoid friction" , "Borehole for petroleum" , "Will Leo? (anag.)" .)