Rugby Union — the French hold sway (4)
I believe the answer is:
rule
'hold sway' is the definition.
'rugby union the french' is the wordplay.
'rugby union' becomes 'ru'.
'the french' becomes 'le'.
'ru'+'le'='RULE'
(Other definitions for rule that I've seen before include "Regulation, reign" , "Regulation, principle" , "Government - measure" , "Govern with straight stick?" , "Regulation, control" .)