Measure the French fever (6)
I believe the answer is:
league
'measure' is the definition.
(league is a kind of measure)
'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 "Association for mutual co-operation" , "... and three sea miles" , "Association - old measure of distance" , "Group of sports clubs; about three miles" , "Association of sports teams to organise matches" .)