Component of blood pressure lowered after some medical attention at first (6)
I believe the answer is:
plasma
'component of blood' is the definition.
(I've seen this before)
'pressure lowered after some medical attention at first' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'at first' suggests taking the first letters.
The first letters of 'lowered after some medical attention' is 'lasma'.
'p'+'lasma'='PLASMA'
(Other definitions for plasma that I've seen before include "Colourless fluid part of blood" , "Part of blood; ionised material" , "Ionised gas; fluid in blood" , "Colourless fluid in which blood cells are suspended" , "Component of TV screen" .)