Dam-building rodent (6)
I believe the answer is:
beaver
'dam-building rodent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beaver that I've seen before include "Lodge-building rodent" , "Furry animal, lives in lodge and builds dams" , "Dam-building rodent" , "Semi-aquatic rodent" , "Lodge-builder" .)