The Collaborative International Dictionary of English v.0.48:

Clutter \Clut"ter\, v. t. [imp. & p. p. Cluttered; p. pr. &
   vb. n. Cluttering.]
   To crowd together in disorder; to fill or cover with things
   in disorder; to throw into disorder; to disarrange; as, to
   clutter a room.
   [1913 Webster]