Son left university at last keen on games (6)
I believe the answer is:
sporty
'keen on games' is the definition.
(enthusiastic about sports)
'son left university at last' is the wordplay.
'son' becomes 's'.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'at last' indicates one should take the final letters.
The final letter of 'university' is 'y'.
's'+'port'+'y'='SPORTY'
(Other definitions for sporty that I've seen before include "Energetic" , "preferring to play" , "Like athletes" , "Describing clothes marked by gaudiness or jazziness" , "Like some cars" .)