A chess defeat: game over for dilettante (7)
I believe the answer is:
amateur
'dilettante' is the definition.
(I've seen this before)
'a chess defeat game over' is the wordplay.
'chess defeat' becomes 'mate' (checkmate).
'game' becomes 'ru' (rugby union).
'over' is a reversal indicator.
'ru' in reverse letter order is 'ur'.
'a'+'mate'+'ur'='AMATEUR'
'for' acts as a link.
(Other definitions for amateur that I've seen before include "Unpaid (sports)" , "Lover" , "lacking professional expertise" , "Layman" , "Dabbler" .)