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 unsure of the 'of one's argument' bit.
'proportion and flair backing up' is the wordplay.
'proportion' becomes 'ratio' (I've seen this in other clues).
'and' says to put letters next to each other.
'flair' becomes 'elan' (synonyms).
'backing up' is a reversal indicator.
'elan' reversed gives 'nale'.
'ratio'+'nale'='RATIONALE'
'the' is the 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" .)