I am so sorry for your shocking loss.
Maybe something like 'Your uncle had a serious illness which he didn't get help for and he died.' Maybe mention that doctors can help with that disease, so she knows from an early age that treatment is available. If she asks 'How did he die?'. I might say 'he hurt himself and he was too hurt to live'. I don't believe I would go into the "how" details. If she is in school, she may have heard of suicide and want information about that. I'd just say something like 'I'm glad to answer your questions', rather than saying "I don't think you are ready". That could add too many ominous overtones and increase her interest in details which she wouldn't be seeking otherwise. Do you follow? My reaction of calm or fear will influence ds's intensity of curiosity, ime.
You might ask in Personal Growth or Spirituality, with a 'trigger warning' on the subject since this is a hard subject to be inadvertently triggered about their own past experiences.

I hope that you will feel welcome to post and seek the experience of folks who can help.
Here is an old post of mine discussing death with our son, around the same age. I told him 'Our body dies, but we have a spirit of energy that stays with the people we love. So our spirit doesn't die, just our body.' I said 'Remember the
Circle of Life? When things die, their body returns to the earth and nourishes the plants and trees, and becomes flowers and air and sun and helps everything that is living. So, the body nourishes everything when it dies. And the spirit is always with the people who loved us. We are always together, even after our body dies. Life is a gift. We live in our body and have lots of fun experiences and then our body dies and nourishes everything and becomes everything. And our spirit is energy and continues in everyone we have ever met and loved.'
http://www.mothering.com/discussions...postcount=1051
I'd focus on the joy of living and the fun times. Certainly, acknowledging missing him and remembering the silly stories and funny memories. Could you find a few favorites of your own to share with her?
Pat