Share a French article in principle (9)
I believe the answer is:
rationale
'principle' is the definition.
('rationale' can be a synonym of 'principle')
'share a french article' is the wordplay.
'share' becomes 'ration' (ration is a kind of share).
'french article' becomes 'le' (french for 'the' - definite article).
'ration'+'a'+'le'='RATIONALE'
'in' acts as a link.
(Other definitions for rationale that I've seen before include "Reasoned exposition" , "This might account for" , "explanation" , "The logical basis of some thing or action" , "Logical reason for action" .)