Half of the school turns out for battle at centre where players hope for a big win (5)
I believe the answer is:
lotto
'battle at centre where players hope for a big win' is the definition.
I can't tell whether this defines the answer.
'half of the school turns out for' is the wordplay.
'half of the school' becomes 'lo' (I can't justify this - if you can you should believe this answer much more).
'turns' becomes 'tt' (an abbreviation for turn written twice).
'out' means to remove the last letter.
'for' becomes 'to' (eg both can mean 'in the opinion of').
'tt' with its last letter taken off is 't'.
'lo'+'t'+'to'='LOTTO'
Can you help me to learn more?
(Other definitions for lotto that I've seen before include "house" , "Children's game, housey-housey" , "Bingo - Venetian painter" , "Game of chance based on numbered balls being drawn at random" , "National game with large money prizes" .)