how do I tell my daughter? - from JN

Hi Prison Widow and fellow readers. I was pointed to your blog by a friend of mine and have lurked in the shadows reading posts until I could respond with my story. I have read the post by Anon who stated that there is no support for prisoners families with a loved one in prison battling with drug addiction. 
I agree and took a deep breath whilst reading it. 
My daughter is now 13 and is asking more indepth questions about her dad who is currently in prison. He has a drug addiction problem and I have no idea how to explain this to her. 
He is a heroin addict but no offence, doesn't look like one meaning he worked, ate well and looked after himself of a fashion. 
I have sheltered my daughter a lot and protected her from the wicked world but now she is growing up and fast and wants answers; answers I am struggling to get across. 
How do you tell a 13 year old that their dad is a drug addict especially one addicted to heroin; a drug that is ruthless? 
Just before Christmas she came home from school and asked me if her dad was a 'smackhead' (her words) because a girl in her class told her he was and that her Mum knows him. 
I can't prolong this anymore so I intend to sit and talk to her this weekend. 
She visits her dad in prison and has asked him why he keeps breaking the law. He dances around the question and doesn't tell her the truth in order not to upset her but she is going to find out the truth one day whether I or her dad tries to protect her from it or not. The post about family visits struck a chord with me because the person is spot on by saying that family visits are somewhat of a farce because the whole thing isn't realistic, well it isn't for me and I trust they aren't for children who's parent in prison is battling with addiction and continues to reoffend. 
I am at a loss. I am so at a loss. 
Please don't publish my real name. Thank you.