Garlic clove has turned to a firm jelly texture I keep my garlic on my kitchen shelf by the window. Out of the whole garlic bulb one the cloves has changed to a yellow/orange colour and its texture is like hard jelly. Its slightly squidgy but firm.

How has this happened? Is it edible?
This is called waxy breakdown.  It's a defect rather than a disease, so you don't have to worry about a microbe or parasite.  My neighbors have used affected cloves and they're still alive, though I've never been able to make myself use them because of the texture.