Burrowing mammals (5)
I believe the answer is:
moles
'burrowing mammals' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for moles that I've seen before include "Small velvety-furred burrowing mammals" , "Small dark spots on the skin" , "Traitorous spies" , "Breakwaters" , "Skin spots or burrowing animals" .)