Tincture used in painting (6)
I believe the answer is:
iodine
'tincture' is the definition.
(iodine is a kind of tincture)
'in painting' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
This may be the basis of the clue (or it may be nonsense).
'used' is the link.
Can you help me to learn more?
(Other definitions for iodine that I've seen before include "Mild antiseptic" , "Dark-violet antiseptic" , "Disinfectant made from seaweed" , "Infamous tablets in case of a nuclear emergency" , "Mother-of-pearl" .)