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 really see how this works, but
'in' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'used' acts as a 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" .)