Compound used in dyeing and tanning (4)
I believe the answer is:
alum
'compound used in dyeing and tanning' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for alum that I've seen before include "Compound used in dyeing and to stop bleeding" , "Name given to a specific chemical compound and a class of them" , "Potassium compound; IoW sandy bay" , "Compound used in tanning and dying" , "Aluminium/potassium sulphate" .)