Complex sentence: Adrian started off as a member of the now defunct group "Seven" as a lead singer in the group he was instrumental in giving the group its unique sound a sound which he has successfully managed to maintain and embody as a solo performer.
Split into simpler sentences: Adrian started off as a member of the now defunct group "Seven". As a lead singer in the group he was instrumental in giving the group its unique sound a sound which he has successfully managed to maintain and embody as a solo performer.