The French following Rugby Union laws (5)
I believe the answer is:
rules
'laws' is the definition.
(rule is a kind of law)
'the french following rugby union' is the wordplay.
'the french' becomes 'les' (plural form of 'the' in French).
'following' says to put letters next to each other.
'rugby union' becomes 'ru'.
'les' put after 'ru' is 'RULES'.
(Other definitions for rules that I've seen before include "Reigns" , "strips" , "Is king" , "Laws; draws lines" , "Regulations, precepts" .)