Complex sentence: Ross (1969), who was the first to examine sluicing, assumed that sluicing involves regular "wh"-fronting followed by ellipsis of the sister constituent of the "wh"-phrase.
Split into simpler sentences: Ross (1969), who was the first to examine sluicing. He assumed that sluicing involves regular "wh"-fronting followed by ellipsis of the sister constituent of the "wh"-phrase.