A friend -- flipping rugby union enthusiast (7)
I believe the answer is:
amateur
'enthusiast' is the definition.
(thesaurus)
'a friend flipping rugby union' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'flipping' is a reversal indicator.
'rugby union' becomes 'ru'.
'ru' in reverse letter order is 'ur'.
'a'+'mate'+'ur'='AMATEUR'
(Other definitions for amateur that I've seen before include "Fancier" , "A non-professional sportsman" , "I'm no expert!" , "Of sportsman, not professional" , "lacking professional expertise" .)