Straight man mostly to halt rise of self-assertiveness (6)
I believe the answer is:
stooge
'straight man' is the definition.
(I've seen this before)
'mostly to halt rise of self-assertiveness' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'to halt' becomes 'stop' ('stop' can be a synonym of 'halt').
'rise of' is a reversal indicator (in a down clue, letters go up).
'self assertiveness' becomes 'ego' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ego' in reverse letter order is 'oge'.
'stop' with its last letter taken away is 'sto'.
'sto'+'oge'='STOOGE'
Can you help me to learn more?
(Other definitions for stooge that I've seen before include "Comedian's assistant-victim" , "Toes go wrong for the scapegoat" , "Butt of a comic's jokes" , "Partner who is the butt of comedian's joke" , "Fall guy" .)