Can someone tell me what the notation "ー・" means?
It's all over this article: 大辞林「いう【言う・云う・謂う】」
It simply means that it should be replaced by 言, 云 or 謂 in actual usage.
The notation is used exclusively in some dictionaries to avoid looking repetitive in giving example phrases/sentences for the entry word.
(Of course, one could argue that the use of the same notation over and over looks repetitive, too.)
This dictionary definition, which you quoted, looks a bit half-baked.
Have a look at 大辞泉 for 話す.
It first defines the root (constant) part:
And then uses it in examples:
Where the part before the dot is constant and the ending is conjugated/inflected.
Back to the example from the question, it should define い・う for the examples to make sense.