Complex sentence: He received little formal schooling and eventually apprenticed himself to a clockmaker, during time Fitch was not allowed to learn or even observe watchmaking (he later taught himself how to repair clocks and watches).
Split into simpler sentences: He received little formal schooling and eventually apprenticed himself to a clockmaker. During his apprenticeship, Fitch was not allowed to learn or even observe watchmaking (he later taught himself how to repair clocks and watches).