Rugby Union name in race (3)
I believe the answer is:
run
'race' is the definition.
(run is a kind of race)
'rugby union name' is the wordplay.
'rugby union' becomes 'ru'.
'name' becomes 'n' (common abbreviation e.g. on forms).
'ru'+'n'='RUN'
'in' is the link.
(Other definitions for run that I've seen before include "Race; flee" , "Uninterrupted sequence" , "athletic event" , "Flow; function" , "escape" .)