Amphibious rodent (6)
(Other definitions for beaver that I've seen before include "Animal with soft fur, builds dams" , "Hard-working furry rodent" , "Semi-aquatic rodent" , "tree-feller" , "Creature that lives in lodge" .)
(Other definitions for muskrat that I've seen before include "Large aquatic rodent" , "UK, smart? (anag)" , "Vole-like animal" , "Rodent prized for its fur" , "Rodent valued for its fur" .)