If he comes while I'm away, please tell him to wait until I return.
Literal
Absence-during [time-に] if he [subject-が] came [conditional-たら], I [subject-が] return [until-まで] wait [manner-ように] say please.
A multi-layered request: ~たら (if), ~まで (until), ~ように言ってください (please tell him to). Three patterns nested inside a single polite request. 留守中に (while I'm away) sets the time frame.