A number book into unbelievable game (10)

'game' is the definition.
(type of ball game)

'a number book into unbelievable' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'number' could be 'l' (L is an example) and 'l' is located in the answer.
'book' could be 'b' (abbreviation for book) and 'b' is present in the answer.
This may be the basis of the clue (or it may be nonsense).