Complex sentence: Keizer then first played as an amateur for Kent side Margate; at the time, Margate acted as a "nursery" club of sort for London giants Arsenal, and soon Keizer was spotted by Arsenal's manager, Herbert Chapman.
Split into simpler sentences: Keizer then first played as an amateur for Kent side Margate. At the time Margate acted as a "nursery" club of sorts for London giants Arsenal, and soon Keizer was spotted by Arsenal's manager, Herbert Chapman.