Tissue sample test (6)
I believe the answer is:
biopsy
'tissue sample test' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for biopsy that I've seen before include "Test on living tissue" , "Microscopic test for disease" , "Test for infection" , "Examination of tissues to determine disease" , "Examination of tissues cut from living body" .)