The reason there is no Claw is because his hands are normal until his "Innocence" is activated. Gaia also requires us to have both arms. When Allen reaches a sync level of over 100 he can activate his Innocence farther and transform his whole arm into a sword(The death whisper.) Further more, during battle his Crown Clown extends into a large cloak as it is now a full body armor type innocence.
References for those of the D.Gray-Man less life.
http://i228.photobucket.com/albums/ee178/wannabeangel4eva/440137-20070728135546.jpg
