Screen for blood component (6)
I believe the answer is:
plasma
I believe this is a double definition.
'screen' is the first definition.
(I've seen this before)
'blood component' is the second definition.
(I've seen this in another clue)
'for' acts as a link.
(Other definitions for plasma that I've seen before include "Yellow liquid in blood" , "element of transfusion" , "Part of blood; ionised material" , "Colourless fluid part of blood" , "Clear fluid portion of blood" .)