The French fever for union (6)
I believe the answer is:
league
'for union' is the definition.
(league is a kind of union)
'the french fever' is the wordplay.
'the french' becomes 'le' ('the' in French).
'fever' becomes 'ague' (I've seen this before).
'le'+'ague'='LEAGUE'
(Other definitions for league that I've seen before include "Old distance measure of about three miles" , "Association for a common purpose" , "Group in football" , "Association of sports teams to organise matches" , "sporting group" .)