Complex sentence: Cleckley explained in the preface that it "grew out of an old conviction", which he does not clarify the origins of but says it "increased during several years while I sat at staff meetings in a large neuropsychiatric hospital".
Split into simpler sentences: Cleckley explained in the preface that it "grew out of an old conviction". He does not clarify the origins of that, but says it "increased during several years while I sat at staff meetings in a large neuropsychiatric hospital".