Assay stamp (8)
I believe the answer is:
hallmark
'assay stamp' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for hallmark that I've seen before include "Distinctive feature" , "Stamp indicating standard of gold etc" , "Metal assay sign" , "Assay-stamp" , "Sign of authenticity (eg of silver)" .)