Old man and old woman receiving large then small quantity of blood (6)
I believe the answer is:
plasma
'blood' is the definition.
(thesaurus)
'old man and old woman receiving large' is the wordplay.
'old man' becomes 'pa' (both can mean someone's father).
'and' says to put letters next to each other.
'old woman' becomes 'sma' (I can't explain this - if you can you should give a lot more credence to this answer).
'receiving' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'pa'+'sma'='pasma'
'pasma' enclosing 'l' is 'PLASMA'.
'then small quantity of' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for plasma that I've seen before include "Clear part of blood" , "Colourless fluid in blood; green variety of quartz" , "Blood component and type of television" , "Colourless fluid in which blood cells are suspended" , "A psalm (anag.)" .)