A tincture used in painting (6)
I believe the answer is:
iodine
'a tincture' is the definition.
(I know that iodine is a type of tincture)
'in painting' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
This explanation may well be incorrect...
'used' acts as a link.
Can you help me to learn more?
(Other definitions for iodine that I've seen before include "Mother-of-pearl" , "Disinfectant made from seaweed" , "Element existing in vapour and crystalline form" , "Solution applied to wounds as an antiseptic" , "Antiseptic made from seaweed" .)