The writer is talking about Nature's rule -you get what you do. Good deeds, good return, bad deeds, bad returns.
If someone thinks that the Nature is just a fruit machine i.e. will keep on giving you everything in spite of you doing nothing good, it is not so. The Nature is not a fruit machine because she'll keep her all credits clean. This means if she keeps her credits clean, she does take care of your account. She maintains her accounts by paying good to those who did good and bad to those who did bad.
The writer has done it for the pun of it by putting the word 'fruit'. As the next line follows that to get good fruits, one must wait but the singer is not so patient to wait for good fruits!