From Longman Dictionary of Contemporary Englisha difficult/hard/good etc onea difficult/hard/good etc onePROBLEMa particular kind of problem, question, story etc ‘What do you attribute your long life to?’ ‘Oh that’s a difficult one.’ → one
Examples from the Corpus
a difficult/hard/good etc one• But what is temperament, and how do we define what is a good one?• I knew there was no sense in trying to do a better one.• Maybe it was a crackpot theory, but it was a good one.• Nevertheless, it was always clear that Schmidt's third term in office would prove a difficult one.• Payno was gleeful, for his idea was a good one.• The Berlin Philharmonic as it exists today may be a happier orchestra, but it is in no way a better one.• Then I became a lead project manager and, I have to say, I was a good one.• The belief that hierarchical organizational structure makes for good business is a difficult one to give up. 