Test for the presence of disease (6)
I believe the answer is:
screen
'test for the presence of disease' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for screen that I've seen before include "Conceal; test" , "Divider to select for suitability." , "Surface on which pictures are projected in cinemas" , "Mask" , "See 13" .)