Crossword Genius

Painting efficiently's a boring source of income? (3,4)

Ross

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.)" .)

I've seen this clue in The Times.
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