French article put forward case: right to leave union (6)
I believe the answer is:
league
'union' is the definition.
(I've seen this before)
'french article put forward case right to leave' is the wordplay.
'french article' becomes 'le' (french for 'the' - definite article).
'put forward case' becomes 'argue'.
'right' becomes 'r' (common abbreviation).
'to leave' is a deletion indicator.
'argue' with 'r' removed is 'ague'.
'le'+'ague'='LEAGUE'
(Other definitions for league that I've seen before include "Group of teams competing over a period" , "Association for a common purpose" , "Alliance - old measure of distance" , "Old unit of distance" , "sporting group" .)