Test for purity (5)
I believe the answer is:
assay
'test for purity' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for assay that I've seen before include "Test ore for content" , "Analyse to find the properties" , "Analyse ore for metal content" , "Test quality of metals" , "Test(ing) of a substance, such as precious metal" .)