From Longman Dictionary of Contemporary Englishthe (only) problem is (that) ...the (only) problem is (that) ...spoken used before saying what the main difficulty in a situation is The problem is, there isn’t enough time. → problem
Examples from the Corpus
the (only) problem is (that) ...• In both cases the problem is that concrete cultural processes, in particular historical locations, are reduced to abstract schemata.• I know, the problem is money.• In other words, it is to argue that the problem is a technical problem which admits of a technical solution.• It doesn't mean that the problem is solved, but there's a starting point to work from.• I think that the problem is even too great for remediation.• For them, the problem is one of trust.• Do you know what the problem is?• When the problem is studied and understood, it should explain a great many doubts and questions. 