Complex sentence: He scored a try on his Wales debut (vs Canada) in June 2005, having only come into the Cardiff Blues side a month earlier, after a starring role in the Grand Slam winning Wales Under 21 side.
Split into simpler sentences: He scored a try on his Wales debut (vs Canada) in June 2005. He had only come into the Cardiff Blues side a month earlier, having starred for the Grand Slam winning Wales U.21 side.