Try cleaning and washing the cage and all the furniture and vines and plants if you have live plants clean them too. Then just open a few windows in its room to circulate the air around and see if that helps
Is it the cage that smells, or the chameleon itself? Mine have a certain smell about them and it's quite nice! I just love the smell of Amy's skin when she's been basking in natural sunlight! It's a wonderful smell - rather like babie's heads but the cham version, lol!