Punishes unruly son for 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)
'punishes unruly son' is the wordplay.
'unruly' is an anagram indicator.
'son' becomes 's' (genealogical abbreviation for son).
'punishes' with letters rearranged gives 'pushines'.
'pushines'+'s'='PUSHINESS'
'for' is the link.
(Other definitions for pushiness that I've seen before include "Self-assertion; ambition" , "Ambitious quality" .)