Microscope sample (5)
I believe the answer is:
smear
'microscope sample' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smear that I've seen before include "Slander - tissue sample used for testing" , "Greasy/sticky mark" , "allegation" , "Spread dirty substance, try to damage reputation" , "sample on slide" .)