Pressure monk to preserve small part of vessel's contents (6)
I believe the answer is:
plasma
'part of vessel's contents' is the definition.
(blood contains plasma)
'pressure monk to preserve small' is the wordplay.
'pressure' becomes 'p' (scientific abbreviation).
'monk' becomes 'lama' (as in the Dalai Lama).
'to preserve' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'lama' enclosing 's' is 'lasma'.
'p'+'lasma'='PLASMA'
(Other definitions for plasma that I've seen before include "The watery part of the blood" , "Hot ionised gas" , "Yellow liquid in blood" , "Component of TV screen" , "A psalm (anag.)" .)