Complex sentence: Collingwood was not just a philosopher of history, but also a practising historian and archaeologist, being during his time a leading authority on Roman Britain: he spent his term time at Oxford teaching philosophy but devoted his long vacations to archaeology.
Split into simpler sentences: Collingwood was not just a philosopher of history but also a practising historian and archaeologist. He was, during his time, a leading authority on Roman Britain: he spent his term time at Oxford teaching philosophy but devoted his long vacations to archaeology.