Colourless fluid part of blood (6)
I believe the answer is:
plasma
'colourless fluid part of blood' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for plasma that I've seen before include "Liquid part of blood" , "The watery part of the blood" , "Colourless fluid in blood; green variety of quartz" , "Component of TV screen" , "Clear part of blood" .)