Willing to join school sports (5)
I believe the answer is:
games
'sports' is the definition.
(I've seen this before)
'willing to join school' is the wordplay.
'willing' becomes 'game' (game can mean willing to do something).
'to join' says to put letters next to each other.
'school' becomes 'S'.
'game'+'s'='GAMES'
(Other definitions for games that I've seen before include "Events collectively" , "Favourite lessons" , "Such as football, snap, etc" , "Sporting contests" , "Sports and pastimes" .)