Complex sentence: Clayton and Bishop revealed they had begun recruitment in Manhattan, despite the potential obstacle of recruits having to pledge allegiance to the British monarch upon joining the RCAF, something that could result in forfeiture of citizenship for the young Americans.
Split into simpler sentences: Clayton and Bishop revealed they had begun recruitment in Manhattan. They faced one big potential obstacle of recruits having to pledge allegiance to the British monarch upon joining the RCAF, as this was something that could result in forfeiture of citizenship for the young Americans.