Proportion and flair backing up the basis of one's argument (9)
I believe the answer is:
rationale
'basis of one's argument' is the definition.
'rationale' can be an answer for 'basis' (I have seen 'Basis of logic' mean 'rationale' so perhaps 'basis' could also mean 'rationale'). I am not sure about the 'of one's argument' bit.
'proportion and flair backing up' is the wordplay.
'proportion' becomes 'ratio' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'flair' becomes 'elan' (synonyms).
'backing up' shows that the letters should be reversed in order.
'elan' backwards is 'nale'.
'ratio'+'nale'='RATIONALE'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "The logical basis of some thing or action" , "Logical reasons (for action)" , "Logical basis (for a course of action)" , "justification" , "Logic, reasoning" .)