Matter about buffs showing self-assertive behaviour (9)
I believe the answer is:
pushiness
'self-assertive behaviour' is the definition.
(I know that pushiness is a type of self-assertive behaviour)
'matter about buffs' is the wordplay.
'matter' becomes 'pus' (liquid matter produced by the body).
'about' indicates putting letters inside.
'buffs' becomes 'shines' (I am not sure about this - if you are sure you should believe this answer much more).
'pus' enclosing 'shines' is 'PUSHINESS'.
'showing' is the link.
Can you help me to learn more?
(Other definitions for pushiness that I've seen before include "Self-assertion; ambition" , "Ambitious quality" .)