The Collaborative International Dictionary of English v.0.48:

Clumper \Clump"er\, v. t. [Cf. G. kl["u]mpern to clod. See
   Clump, n.]
   To form into clumps or masses. [Obs.]
   [1913 Webster]

         Vapors . . . clumpered in balls of clouds. --Dr. H.
                                                  More.
   [1913 Webster]