From Longman Dictionary of Contemporary Englishhave an idea (that)have an idea (that)SUREto be fairly sure that something is true, without being completely sure I’m not sure where my necklace is, but I have a pretty good idea who took it. → idea
Examples from the Corpus
have an idea (that)• We can have ideas of things we have not experienced.• This is because I have ideas.• But whenever I have an idea, I need to act on it as soon as possible.• I have an idea of her.• Some have ideas for lyrical language.• We need to have an idea of what perceptions we are triggering. 141 selling Selling is one stage further than communication.• We write the first two chapters together so we have an idea of the characters.• Now that we have an idea how hyperinflation gets started we can look at the causes of run-of-the-mill inflation. 