Complex sentence: Smiley quickly grasps that Tiu's (Ko's second-in-command) quick trip to Shanghai six weeks before Ricardo's planned flight to Red China was to meet with Nelson to arrange their "rendezvous," via which Nelson would escape China.
Split into simpler sentences: Smiley quickly grasps the implication of Tiu's (Ko's second-in-command) quick trip to Shanghai six weeks before Ricardo's planned flight to Red China. It was to meet with Nelson to arrange their reunion "rendezvous," via which Nelson would escape China.