Dam-builder (6)
I believe the answer is:
beaver
'dam-builder' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for beaver that I've seen before include "Broad-tailed rodent" , "tree-feller" , "Soft furry rodent" , "Hard worker (could also be eager)" , "Hard-working furry rodent" .)