Oh no. I left the expensive flour I just bought on the train.
Small wheat powder is wheat flour, or flour.
This word uses the kun'yomi readings. The 小 portion uses a secondary kun'yomi reading for it. Just think about how "small" things are like "children" (子), which is why it's こ. You should have learned the other two readings already, so this should be a piece of flour-filled cake.