Component of blood pressure lowered after some medical attention at first (6)
I believe the answer is:
Here is my best explanation:
'component of blood' is the definition.
'pressure lowered after some medical attention at first' is the subsidiary indication.
'pressure' becomes 'p' (abbreviation).
'at first' indicates taking the first letters.
The first letters of 'lowered after some medical attention' is 'lasma'.
(Other definitions for plasma that I've seen before include "Colourless watery fluid of the blood", "Watery part of blood", "Blood component and type of television", "Clear fluid portion of blood", "Colourless fluid in which blood cells are suspended", "Watery fluid of blood", "Component of blood", "Hot ionised gas", "The watery part of the blood" and "Clear part of blood".)