Painting has the power that a mechanic could make good use of (6)
I believe the answer is:
oilcan
'that a mechanic could make good use of' is the definition.
(a mechanic might use an oilcan)
'painting has the power' is the wordplay.
'painting' becomes 'oil' (oil painting is a kind of painting).
'has the power' becomes 'can' (if you can do something, you have the power to).
'oil'+'can'='OILCAN'
(Other definitions for oilcan that I've seen before include "Lubricant applicator" , "Lubricator with a long nozzle" , "Vessel for viscous liquid" , "Lubricant container" , "Vessel for lubricant" .)