Blood drive participant (5)
I believe the answer is:
donor
'blood drive participant' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for donor that I've seen before include "A giver, often of blood" , "One who gives, maybe blood" , "Organ giver?" , "Almsgiver, benefactor" , "One who gives something" .)