Sample on a slide (5)
I believe the answer is:
smear
'sample on a slide' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smear that I've seen before include "allegation" , "Calumniate" , "Smother" , "Besmirch" , "Slanderous denigration of character" .)