Hospital fluid (5)
I believe the answer is:
serum
'hospital fluid' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for serum that I've seen before include "Watery liquid as from blister, or fluid in blood" , "Watery fluid from animal tissue" , "Constituent of blood" , "Inoculation liquid" , "Watery part of blood" .)